Mental Rotation of Directional Tactile Stimuli

Brian T. Gleeson and William R. Provancher

Haptics and Embedded Mechatronics Lab, University of Utah

ABSTRACT

Several researchers have developed haptic devices capable of rendering directional stimuli. When these devices are integrated into mobile or handheld devices, it becomes possible for a user to hold the haptic device in any orientation and thereby receive directional stimuli that may be out of alignment with rest of the world. In such cases, it becomes necessary for the user to perform a mental transformation of the directional stimuli, so that the stimuli may be understood in a fixed or global reference frame. This paper addresses two questions: 1. can users perform such transformations and successfully interpret stimuli, and 2. what cognitive processes are involved in these transformations? In our experiments, users performed timed identification of directional tactile stimuli with their hand in a variety of orientations around a single axis. The results show that: 1. users can successfully identify directional stimuli both quickly and accurately, even when the stimuli are rendered in a rotated reference frame, and 2. these tasks involve the mental rotation of a spatial mental representation of the stimulus, and also show evidence of embodiment effects. Furthermore, small angles of rotation (up to ~40°) incur very little cognitive cost, suggesting that tactile direction stimuli delivered through a handheld device would be robust to variations in user hand orientation.

KEYWORDS: Directional Skin Stretch, Shear Feedback, Mental Rotation.

Index Terms: H.1.2 [Models and Principles]: User/Machine Systems--Human information processing; H.5.2 [Information Interfaces and Presentation]: User Interfaces—Haptic I/O

1 INTRODUCTION

Haptic direction cues are important because they have the

potential to improve safety and enhance the user experience for a

range of devices. For example, drivers in traffic, soldiers in

combat, and emergency workers in a disaster area must all devote

their visual and auditory attention to their surroundings in order to

maintain situation awareness and personal safety. In such

situations, a haptically-enabled device could provide important

directional or navigational information while leaving the user’s

eyes and ears free. In situations of visual and auditory information

overload, haptic communication can provide cognitive advantages

[1]. For all users, and particularly for the blind, haptic interfaces

could provide an unobtrusive means of receiving information

from common devices like portable phones or music players. As

haptic displays are integrated into mobile or handheld devices, it

becomes necessary to address the issue of mental transformation

of haptic stimuli.

In this paper, we address an important problem inherent to

many types of haptic communication: the mental transformation

of haptic stimuli between different reference frames. In common

laboratory evaluations of directional stimuli, the haptic device is

generally placed in the most natural orientation, that is, aligned

with some global reference frame. In application, however, it may

not be possible to guarantee such alignment, particularly as one

considers handheld or wearable devices.

As an example of this problem, consider navigational

information delivered to the fingertip by a portable device. The

device would render a direction stimulus on the fingertip (in the

reference frame of the finger) and the user would have to interpret

that information as it relates to his or her surroundings (in the task

space). If the user’s finger were not aligned with the task space, it

would be necessary for the user to mentally perform some spatial

transformation between the haptically-perceived reference frame

and the task-based, or global, reference frame (Fig. 1).

Any time there exists a difference between the haptically-

perceived reference frame and the task-based reference frame in

which the cues are to be interpreted, some mental spatial

transformation will be required. Such transformations could have

a serious impact on the applied use of haptic stimuli; previous

work conducted on mental rotation of visual objects has shown

that these rotations incur a cognitive cost and can burden spatial

working memory [2], potentially interfering with other spatial

tasks. With the large and growing number of handheld or

fingertip-based direction displays, it is important to study the use

of such devices in situations where mental transformation is

required.

As an initial investigation of the mental transformation of

haptic stimuli, this paper addresses two questions: 1. Can users

successfully interpret directional stimuli when the orientation of

the hand changes and a transformation between reference frames

is required? 2. What are the cognitive processes underlying the

transformation of haptic stimuli between reference frames?

Fig. 1 When using a portable haptic interface, it may be necessary

to mentally transform haptic cues from the finger-centered

frame, where they are perceived, to the world-centered

frame, where they are to be applied.

————————————————

Department of Mechanical Engineering, University of Utah, Salt Lake City, UT, 84112. E-mail: brian.gleeson@gmail.com, wil@mech.utah.edu.

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IEEE Haptics Symposium 20124-7 March, Vancouver, BC, Canada978-1-4673-0809-0/12/$31.00 ©2012 IEEE

2 BACKGROUND

2.1 Accurate Perception of Rotated Stimuli

Prior research suggests that one’s ability to accurately interpret

directional stimuli may be affected by the orientation of the hand.

In studies of orientation perception, Kappers et al. found variable

hand orientation to cause systematic errors in the perception of

stimulus orientation [3]. The effects of hand orientation also

impact spatial processing and the ability of people to mentally

rotate stimuli between reference frames [4]. The adverse impacts

of variable hand orientation are also demonstrated in studies of

haptic perception and comparison [5] and haptic control of virtual

reality environments [6]. These results warn that variable hand

orientation could potentially interfere with the accurate perception

of haptic direction cues, thus motivating our current study.

2.2 Cognitive Processes Underlying Mental Transformation

The question of how we mentally transform stimuli is ultimately a

question how those stimuli are represented in the mind. A

directional stimulus could be represented in two ways:

symbolically or spatially. For example, if a haptic stimulus were

delivered to the fingertip indicating the direction left, this could be

represented in the mind symbolically, like the word ‘left’, or

spatially, like a vector pointing in a given direction. How

directional stimuli are represented is significant because spatial

representations can be subjected to continuous transformations,

while symbolic representations cannot.

The cognitive science literature contains a large body of work

addressing the mental transformation of stimuli and how those

stimuli are represented in the mind, although the majority of this

work discusses only visual stimuli. Speaking generally,

researchers have found that mental rotation of stimuli is complex

and greatly affects both the cognitive load of a task and the time

required to complete the task [7], while mental translation has

fewer clear costs [8]. As such, our initial investigations, presented

in this paper, will focus only on mental rotation of stimuli.

Many researchers have addressed mental rotation of visual

stimuli, beginning with a study showing that the time required to

complete a mental rotation task increases linearly with the

required angle of rotation [7]. This was understood to mean that

the participants were processing the stimuli using analog, spatial

mental representations (as opposed to symbolic representations)

and that they were performing the rotation task using a continuous

mental transformation.

Later studies, of greater relevance to haptic research, have

addressed the mental rotation of images of body parts (hands, feet,

etc.). These experiments have confirmed earlier results, showing

spatial representations and continuous mental transformations

(e.g., [9], [10]). In contrast to studies of abstract images (e.g., [7]),

experiments involving images of body parts show embodiment

effects, where participants mentally simulate moving the test

image as if the depicted hand or foot was a part of their own body.

Neural imaging experiments confirmed these results, showing that

the somatosensory system engages in motor simulation of body-

relevant rotations (e.g., [11] [12] [13]). In our experiments, which

involve different physical orientations of the hand, embodiment

effects play a significant role.

The few studies addressing mental rotation of tactile stimuli

have generally produced results similar to those obtained in visual

studies. Most of these experiments have involved an embossed

shape pressed against an unmoving fingertip, such as

alphanumeric characters [14] [15] [16] or abstract shapes [17]

[18]. A study where participants felt models of human hands also

showed evidence of spatial representation and mental rotation

[19]. Other studies, however, have produced contradictory results.

A study using vibrating pins failed to produce evidence of mental

rotation [20], and experiments with Braille-like dots only showed

mental rotation behavior under certain conditions [21]. These

differing results show that both the stimulus type and the nature of

the experimental task can determine how the stimulus is

represented in the mind and how the participant performs the

rotation task. In all previous haptic studies, participants perceived

static shapes, so it remains uncertain how participants will

perform mental transformations in tasks involving active,

directional stimuli. To the best of our knowledge, the

transformation of active, directional stimuli is addressed for the

first time in the present study.

2.3 Prior Work with Directional Stimuli

Haptics researchers have developed a range of methods to

communicate directional information using tactile stimuli. Arrays

of vibrating motors can successfully communicate direction cues

and have been built into wearable devices including vests [22] and

belts [23]. Vibrotactile cues have also been delivered through a

chair [24] or a steering wheel [25]. Examples of hand-held or

fingertip-based devices include those that use inertial forces to

communicate direction [26], and others which use shear forces,

slip, or skin stretch at the fingertip (e.g., [27], [28] [29] [30]). In

our previous work, we evaluated the uses of directional tangential

skin stretch at the fingertip. We found this method of tactile

communication to be highly effective; when the skin of the

fingerpad was displaced 1 mm at 2 mm/s or faster, in 1 of 4

cardinal directions, participants were able to identify the direction

of the stimulus with better than 99% accuracy [31]. In all previous

studies of directional tactile stimuli, the stimulus reference frames

were aligned with the response reference frames. In the present

study, we investigate the use of tactile stimuli in cases where the

participant must transform the stimuli between two rotated

reference frames.

3 GENERAL METHODS

In an experiment addressing mental rotation around a single axis,

participants perceived a directional tactile stimulus on the index

finger of their right hand and were asked to indicate the direction

of the stimulus using a joystick held in the left hand. The joystick

and left hand were aligned with the allocentric (world-centered)

reference frame, while the right hand and the tactile stimulus were

rotated into a various fixed orientations. That is, stimuli were

perceived in a various reference frames (right hand), and were

interpreted in a constant reference frame (left hand). The

foundations for our experiment were first established through a

pilot study.

3.1 Participants

The experiment was completed by 15 volunteer participants, 10

male, 5 female, aged between 19 and 33 years (mean = 25.9).

3.2 Tactile Stimulus

Our study utilized directional skin stretch as a tactile stimulus. A

custom haptic device stretched and displaced the skin of fingerpad

in a plane tangent to the surface of the fingerpad (Fig. 2). With the

user’s finger held stationary, the haptic device moved a rubber

contact element that was pressed against the skin of the finger,

stretching the skin in one of four cardinal directions. The haptic

device, described in detail in [32], consisted of two servomotors, a

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flexure stage that converts the rotational motion of the servos to

linear translation, a rubber contact element, and a thimble-type

finger restraint. The contact element which delivered the stimulus

to the fingertip was a ThinkPad TrackPoint (7 mm diameter

rubber hemisphere with sandpaper-like surface finish).

For each stimulus, the contact element completed a 1±0.1 mm

out-and-back motion, as shown in Fig. 3, in one of four cardinal

directions: proximal, distal, radial or ulnar. We limited the stimuli

to only four cardinal directions (90 degree increments) in order to

eliminate the confounding factor of precise stimulus angle

perception. The perception of the four-direction stimuli used in

this experiment is well understood; previous work has shown the

stimuli to be highly salient and to unambiguously communicate

direction. In earlier studies, subjects were able to correctly

identify the direction of stimuli with 99+% accuracy. For detailed

information on stimulus rendering and perception, see [31, 32].

The full perceptual nature of the stimulus is somewhat

complex, consisting of the primary skin stretch but also small

amounts of slip between the finger and contact element, as well as

the contact between the finger and the restraining thimble. The

stimulus is discussed in greater depth in our previous work, but

for the present experiment it is only important that the stimulus

provide unambiguous direction cues, as it has been shown to do

[31, 32]. Although we test only one stimulus type, we consider the

current work to apply broadly to the various types of directional

tactile stimuli discussed in Section 2.3; the skin stretch stimulus

used in our experiment is a representative example of a larger

class of haptic stimuli.

3.3 Apparatus

Test participants sat as shown in Fig. 4(left), with the tactile

display device worn on the right index finger. Wooden fixtures,

fastened to the test surface with hook-and-loop adhesive tape,

determined the location and orientation of the tactile device and

the participant’s hand. After receiving a tactile stimulus,

participants would indicate their response using a four-direction

joystick operated with their left hand. The joystick only allowed

the participant to respond in four cardinal directions so as to

eliminate any confusion arising from ambiguous responses.

Participants received instructions from a monitor positioned in

front of the test apparatus. A PC running Matlab and the

Psychophysics Toolbox [33] controlled the tactile device and

recorded participants' responses with ±1.5 ms timing accuracy.

During the experiment, participants wore headphones playing

white noise and the test environment was covered so that

participants were not able to see their hands.

4 PILOT STUDY

4.1 Pilot Study Procedure

A pilot study was conducted to characterize the natural response

to rotated stimuli. In this study, participants were not instructed

how to map between the stimulus and the joystick. They were told

to respond “…in the direction that you feel best corresponds with

the tactile stimulus.” The pilot study tested a range of pre-defined

hand orientations produced by combining 90º rotations of the

wrist, elbow and index finger (including the 0º and 90º positions

in Fig. 4(right)). Responses were not timed.

4.2 Pilot Study Results

The results of the pilot study show that there is no single, intuitive

mapping between reference frames in our experiment. Participant

responses showed two different mapping patterns: a finger-

aligned mapping and a static, world-aligned (allocentric)

mapping. In the finger-aligned mapping, participants interpreted

the stimuli in the reference frame in which they were rendered.

For example, a distal stimulus on the index finger mapped to the

forward direction on the joystick, even as the finger moved out of

alignment with the joystick. In world-aligned mapping,

participants responded in the direction most closely aligned to the

absolute, physical direction of the stimulus. For example, a

stimulus in a forward direction (away from the body) always

mapped to the forward direction on the joystick, regardless of the

orientation of the stimulus on the finger. Participant responses

were divided approximately evenly between the two mapping

patterns.

From these results we learn that there is no single intuitive

mapping pattern. This result has two important implications for

our primary experiment: first, participants must be trained to

respond in the desired manner, and second, that mapping stimuli

between reference frames is not an unambiguous, automatic

operation.

The presence of multiple mapping patterns forces us to address

the following question: if we are researching the ability of users to

correctly interpret directional stimuli, which mapping pattern do

we consider ‘correct’? We chose finger-aligned mapping for two

reasons: 1. Finger-aligned mapping is most practical for real-

world applications, as it does not require the interface device to be

capable of tracking its orientation with respect to the world-

aligned reference frame, and 2. Asking study participants to map

between the finger-aligned frame and the world-aligned frame

allows us to most directly investigate the problem of mental

rotation and is in accordance with earlier work on this subject

(e.g., [14] [15] [16] [17] [18]).

In our main experiment, presented below, we instructed and

trained participants to use a finger-aligned mapping. This study is

a first effort at understanding the mental rotation and mapping of

tactile direction cues.

Fig. 2. Photo of tactile display with printed circuit board (PCB)

control electronics.

Fig. 3 Tactile stimulus motion profile. The contact element moves

1 mm at approx. 4 mm/s, stretching the skin of the fingerpad.

After a pause of 0.3 s, the contact element returns to the

center position.

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5 MAIN EXPERIMENT

5.1 Main Experiment Procedure

Our main experiment investigated rotation of tactile stimuli

around a single axis by measuring the time required to interpret

directional stimuli while the hand was placed in a variety of

orientations. Participants were tested on six hand orientations

produced by rotating the forearm about the elbow in the horizontal

plane, as shown in Fig. 4(right). Hand orientations were spaced

every 18° between 0° (right arm extended straight forward) and

90° (right arm pointing to the left). This range of hand orientations

was selected as the most practical means of investigating the

simple case of rotation about a single axis.

Participants were instructed to interpret the stimuli in the

reference frame of the fingertip and respond in the allocentric

(joystick) reference frame. That is, a distal stimulus on the

fingertip was to correspond with a forward response on the

joystick, regardless of the orientation of the finger. Participants

were instructed to respond as quickly as possible. A training

session before the experiment ensured that participants understood

the task and could respond with high accuracy.

Participants responded to 24 repetitions of each of the 4

stimulus directions in each of the 6 hand orientations, for a total of

576 stimuli. The presentation order of hand orientations was

balanced between participants. To minimize the effects of

learning or fatigue, each experiment tested each hand orientation

twice, with the stimulus repetitions evenly divided between the

two blocks. That is, participants responded to 48 stimuli in a given

orientation and then moved on to the next orientation, cycling

through all orientations twice. The order of the stimulus directions

was pseudorandom, with an equal number of stimuli presented in

each direction. Participants required an average of 30 minutes to

complete the experiment.

5.2 Data Analysis

Data analysis focused on relative response times (relative RT).

Response time (RT) is the time elapsed between the onset of the

stimulus and the participant’s joystick response. Relative RT is

(RT – baseline RT) where baseline RT is the average response

time in the baseline condition (0°, i.e., with the index finger

aligned with the joystick). Baseline RT values were calculated

individually for each participant. The use of relative RT in the

analysis eliminates baseline differences between participants and

makes the effect of hand orientation more apparent. Data were

pooled for each subject, producing a mean response time for each

hand orientation. The final analysis considered the resultant

collection of subject means.

All incorrect responses were rejected from the data set

(3.0% of data) along with outlier RT values (> 3 standard

deviations from the subject’s mean for a given orientation, 2.1%

of data). Additionally, all data from one participant were rejected,

due to the participant’s unusually high error rate (>3 standard

deviations above the group mean).

5.3 Direction Cue Error Rate and Reaction Time

Our experiment used response time (RT) as a measure of task

difficulty. As a precursor to our main data analysis, we tested the

correlation between RT and error rate to ensure that the

interpretation of our data was not confounded by speed-accuracy

trade-off effects (c.f. [19]). This analysis of correlation addresses

the following question: do larger RTs accurately indicate greater

task difficulty, or are participants merely choosing a different

point on the speed-accuracy continuum for each finger

orientation? The data show a positive correlation between RT and

error rate, the opposite of what one would expect in the case of a

speed-accuracy trade-off. The positive correlation, as measured by

Pearson’s r, is statistically significant (r = 0.94, p < 0.001). This

positive correlation indicates that, under some conditions, the

participants were making more errors even though they were

spending more time on the task, implying that the difficulty of the

task was not constant. From this we conclude RTs may be used as

a measure of task difficulty.

5.4 Accurate Perception of Directional Stimuli in Rotated Reference Frames

Participants responded quickly and acutely to the experimental

stimuli over the range of tested angles. The mean accuracy was

97% and the mean absolute response time was 0.606 s. These

results show that users can interpret directional stimuli quickly

and accurately, even in cases where the reference frame of the

stimuli is rotated with respect to an absolute reference frame.

From this we conclude that directional tactile stimuli may be used

effectively in cases where the haptic device moves with respect to

the environment, such as would be the case with a handheld

device.

5.5 Cognitive Processes Underlying Mental Transformation

A further analysis of our data addresses the question of how users

transform stimuli between reference frames. This analysis focuses

on the relationship between rotation angle and relative RT (there

were too few errors to perform any meaningful analysis of error

patterns). We hypothesized that the directional stimuli used in our

experiment would produce a spatial mental representation, and

that the experimental task would require a mental rotation of this

spatial representation. Based on previous studies (e.g., [7], [9],

[10]), we expected that a spatial mental representation and mental

rotation would be indicated in the data by an increase in relative

RT as a continuous function of the angle between finger frame

and the joystick (world) frame.

Fig. 5 summarizes our results and shows a clear correlation

between rotation angle and relative RT. That is, as the angle

between the finger and joystick reference frames increased,

participants required more time to respond. This correlation is

statistically significant (Pearson’s r=0.602, p<0.0001). This

increase in time implies increased task difficulty and a greater

amount of cognitive processing for greater rotation angles.

From this correlation, we conclude that participants were

forming a spatial representation of the stimuli and performing a

mental rotation, rotating the tactile stimuli from the fingertip

Fig. 4 Left: The test environment. Repositionable fixtures provided

for repeatable positioning of the hand and finger. During

experiments, the test environment was covered with a

wooden lid so that participants could not see their hands.

Right: The six hand orientations tested.

174

frame to the joystick frame (cf., [7]). This result is somewhat

surprising; given the simple stimulus used in our experiment (4

cardinal directions), it would have been reasonable to assume that

participants would perform the experimental task using a simple

symbolic strategy, e.g., by memorizing the relationship between

stimuli and responses, similar to a mental lookup table. This was

not the case, however. A non-spatial mental representation of

stimuli would not be sensitive to the spatial orientation of the

finger. Based on the findings in the large body of prior mental

rotation research, the observed correlation between rotation angle

and response time indicates a mental rotation process based on a

spatial representation of the stimuli (e.g., [7], [9], [10]).

Further insight into the cognitive processes underlying the

experimental task can be gained by considering the shape of the

RT-angle relationship. Our data show a non-linear relationship

between rotation angle and RT, which we fit with a sinusoidal

curve (R2 = 0.41). Other non-linear curves shapes, e.g., a

parabola, fit similarly well, but a sinusoid was chosen as the most

physically relevant model, following the example of a prior study

which also addressed tactile sensing and the mental rotation of

hands [19]. Note that curves were fit to the collection of all

participant means, (6 data points per participant), while Fig. 5

shows pooled means only, for clarity.

The non-linear shape of our data allows us to place our results

in context with earlier studies, suggesting the presence of

embodiment effects in our study. Prior studies of mental rotation

involving abstract shapes generally show linear trends (e.g., [17],

[15], [18]). Non-linear and sinusoidal trends, qualitatively similar

to our results, most often appear when participants exhibit

embodiment effects during the mental rotation of visually or

tactilely perceived human hands (e.g., [34], [35], [19], [10]) or

while controlling an object through physical hand rotation [6].

The implication, therefore, is that participants in our study were

performing some sort of embodied rotation, that is, mentally

rotating their hand into the baseline position (0° position), and

then interpreting the stimulus in that orientation. This result is

important because it allows the classification of our results into

the larger body of past research, shedding light on the cognitive

processing underlying the interpretation of rotated stimuli and

indicating a direction for future work. Because of the presence of

embodiment effects, mental rotation of tactile stimuli may not be

a matter of simple spatial transformation, but may also incorporate

physiological factors, such as the pose of the hand or the angle of

specific joints. A deeper investigation into the effect of hand pose

on mental transformation is the subject of ongoing research.

6 CONCLUSION

Our results suggest that directional haptic cues could be used

effectively in applications where some mental rotation would be

required. Despite the combined difficulties of the mental rotation

task and the skewed perceptual reference frames at rotated finger

orientations, participants were able to identify the direction of the

tactile stimuli quickly and with high accuracy.

The relationship between finger orientation and RT shows that,

even with simple four-direction stimuli, participants mentally

processed the stimuli using analog spatial representations. The

shape of the time-angle curve is of relevance to haptic interface

designers; rotations of small angles (say, 0-40°) can be executed

without great cost, but larger rotations may impede performance.

Additionally, the non-linear nature of the time-angle curve

indicates the presence of embodiment effects, suggesting a

possible relationship between the specific pose of the hand and the

difficulty of the mental transformation.

The results of the present study have the potential to impact a

broad range of haptic applications. Many haptic devices have been

designed to convey directional information. When these devices

are integrated into a mobile or handheld device, users will be

forced to contend with haptic stimuli rendered in the variable

reference frame of the hand. A thorough understanding of the

mental transformation of haptic stimuli will help engineers and

designers to develop haptic interactions that are suitable for

mobile and handheld applications.

Based on the results of this study, we propose the following

guidelines for the design of haptic devices and interactions: 1.

Designers may produce devices and interactions that would

require users to successfully interpret spatial stimuli presented in a

rotated reference frame, although, 2. The interpretation of such

stimuli is not unambiguous and users will have to be instructed on

proper interpretation. 3. Expect that the interpretation of these

stimuli will come at the cost of longer reaction times and higher

cognitive demands for the user, but, 4. These costs can be

minimized if devices and interactions are designed to prevent

reference frame rotations from exceeding approximately 40

degrees.

Our ongoing work continues to address the problem of rotated

haptic stimuli. Current experiments are addressing the question of

embodiment and the impact of specific hand poses on the

interpretation of haptic cues.

ACKNOWLEDGEMENTS

We thank Rebecca Koslover for her hard work in preparing and

proctoring the experiments. Thanks also to Dr. Astrid Kappers for

her advice and comments. This work was funded, in part, by the

US National Science Foundation under grant # IIS-0746914.

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