Perception and the Medial Temporal Lobe: Evaluating the Current EvidenceWendy Suzuki

Introduction• Medial Temporal Lobe Memory System (MTLMS)

Hypothesis▫ Subset of structures of MTL underlies learning and

declarative/relational memory Structures: hippocampus, parahippocampal gyrus

(entorhinal cortex, perirhinal cortex, parahippocampal gyrus)

▫ Not involved in perception

• Perceptual-Mnemonic Hypothesis (of MTL function)▫ MTL involved in certain forms of high-level perception

Perirhinal cortex: visual object perception (feature ambiguity)

Hippocampus: perceptual processing of visual scenes

Aim and Argument• Aim

▫ Evaluation of evidence supporting the perceptual-mnemonic hypothesis of MTL function

• Argument▫ Evidence is not sufficient to support the notion that

MTL plays an important role in perception

Evidence for the Medial Temporal Lobe Memory System

Evidence for the Medial Temporal Lobe Memory System:Patient H.M.

• Age 27• Suffered from Epilepsy• Treatment:

▫ Bilateral resection of MTL, including anterior hippocampus (with atrophy to posterior part), dentate gyrus, parahippocampal gyrus (parahippocampal, entorhinal, & perirhinal cortices), and amygdala

• Resulting behavioural changes:▫ Reduction in epileptic seizures▫ Severe, long-lasting memory impairment

Selective to declarative memory for facts and events Procedural and implicit memory left intact

Evidence for the Medial Temporal Lobe Memory System:Animal Studies• Animal model of human amnesia confirmed

behavioural observations in humans▫ Memory was severely impaired▫ Perceptual abilities spared

• Mishkin (1978)▫ 1st researcher to replicate H.M.’s lesions in animals▫ Resulted in profound deficits on the delayed

nonmatching-to-sample task

Delayed Nonmatching-to-Sample Task

Evidence for the Medial Temporal Lobe Memory System:Conclusions• Evidence from human neuropsychological

studies and animal lesion studies supports the MTLMS hypothesis▫ MTL plays a critical role in declarative/relational

learning and memory E.g. Associative learning/memory, recognition memory

over long delay intervals

▫ MTL is NOT a necessary structure for the online perceptual processing of sensory information E.g. Features of sensory stimuli, differentiating

between stimuli

Evidence for the Medial Temporal Lobe Memory System:Conclusions

▫ Higher-order sensory information from sensory/ association areas projects to MTL Information is associated together & used for learning

& memory However, MTL does NOT provide additional

perceptual processing

• Perceptual-Mnemonic Hypothesis▫ MTL uniquely involved high-level forms of

perception

Evidence from Animal Lesion Studies

Perirhinal Lesions in MonkeysEacott et al (1994)• Bilateral lesions to entorhinal & perirhinal

cortices• Difficult matching-to-sample task (see Figure)

▫ Monkeys with lesions had more errors than controls

• Delayed nonmatching-to-sample task▫ Monkeys with lesions had

more errors than controls

• Results from the difficult matching task were called into question by Buffalo & colleagues because some data left out of analyses

Perirhinal Lesions in MonkeysBuckley et al (2001)• Oddity discrimination task

▫ Selectively targets perception and not memory

Buckley et al 2001

Perirhinal Lesions in MonkeysBuckley et al (2001)• Oddity discrimination task

▫ Selectively targets perception and not memory▫ Monkeys with PC lesions do not differ from controls

for simple or moderately complex discriminations▫ Monkey are impaired when complexity of

discrimination increases▫ Taken as evidence for role of PC in higher-level

perception

▫ Problem: fails to account for contributions of associative learning and LT memory

Perirhinal Lesions in MonkeysBuckley et al (2001)• Associative learning:

▫ Must associate or hold 3 similar faces in memory

▫ Thus, may be impaired visual associative learning, rather than impaired visual perception

▫ Robust evidence in literature supporting role of perirhinal cortex in associating visual stimuli together in memory (see Messinger et al 2001; Murray et al 1993; Sakai and Miyashita 1991)

Buckley et al 2001

Perirhinal Lesions in MonkeysBuckley et al (2000)• Long term memory:

▫ Monkey had prior exposure to all stimuli

▫ Control monkeys may benefit from previous exposure, developing long-term memory for the faces

▫ Thus, impaired associative learning or LT memory for individual scenes vs. impaired visual perception may explain deficits seen

Buckley et al 2001

Perirhinal Lesions in MonkeysBussey et al (2003)• 1st task: Monkeys performed a

simple visual discrimination task• 2nd task: Pictures were morphed to

have more feature overlap• Monkeys with PC lesions

significantly impaired on high feature overlap items

• Problem: must compare the high feature overlap stimulus to the original target stimulus held in memory

Bussey et al 2003

Perirhinal Lesions in MonkeysBaxter (2009) on Bussey et al (2002)• Task involved discrimination for low,

intermediate, or high levels of feature ambiguity▫ Monkeys with PC lesions showed greatest

impairment for learning stimuli with high feature ambiguity

▫ Demands on learning and memory constant across conditions

▫ Interpretation: cannot attribute deficits to learning or memory impairments

▫ Problem: repeated exposure to the same features increases memory for those features

Perirhinal Lesions in MonkeysCONCLUSIONS• Evidence for the role of perirhinal cortex in

perception is unconvincing▫ Experimental paradigms fail to isolate perceptual

demands

Evidence from Human Neuropsychological Studies

Perirhinal Lesions in HumansStark & Squire (2000)• Tested amnesic patients using oddity

discrimination task▫ No impairment on any tasks▫ Argument: when memory for task instructions is well

controlled, no impairment on perceptual tasks

• Lee et al (2005b) repeated the study, but increased perceptual difficulty▫ Increased stimulus set size▫ Incorporated a trial-unique task▫ 3 MTL patients impaired on the face/scene oddity

tasks; 4 hippocampal patients impaired on trial unique scene task

Lee et al 2005b

Perirhinal Lesions in HumansLee et al (2005c)• Tested MTL-damaged and hippocampal-damaged

patients on difficult visual discriminations

Lee et al 2005c

Lee et al 2005c

Perirhinal Lesions in HumansLee et al (2005c)

Perirhinal Lesions in HumansShrager et al (2006)• Replicated both tasks by Lee and colleagues

(2005c)

Perirhinal Lesions in HumansShrager et al (2006)• Replicated both tasks from Lee and colleagues• Experiment 3:

▫ Trial unique version of the visual discrimination task in experiment 2, but greater feature overlap

Perirhinal Lesions in HumansConflicting results?• Methodological differences

▫ Differences in task design might account for discrepant results Explains differences between Stark & Squire and Lee et al Does NOT explain differences between Lee and Shrager

• Brain Damage▫ Shrager patients: 2 MTL patients had extraneous

damage to anterior temporal polar cortex, anterior insula, and fusiform gyrus

▫ Lee patients: 3 MTL patients had similar extraneous damage as above, with 1 patient also having damage to lateral temporal cortex

Perirhinal Lesions in HumansConflicting results?• Accuracy of estimating brain damage

▫ Subjective rating based on visual inspection using a 4 or 5 point scale

▫ Only considers 9 temporal lobe regions Large areas of tissue unexamined!

Conclusions• Recall: MTLMS hypothesis

▫ MTL involved in declarative/relational learning and memory▫ Not involved in perception

• Evaluating the Evidence: MTL involved in perception?▫ Unconvincing thus far▫ Animal studies limited by difficulty in parsing out memory

from perception▫ Human studies limited by measurement and reporting of

brain damage

• Concluding Argument: MTL is involve in declarative/relational learning and memory with little or no involvement in perception

Questions?