Perception and the Medial Temporal Lobe: Evaluating the Current Evidence Wendy 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) ▫1 st 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 Monkeys Eacott 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 Monkeys Buckley et al (2001) Oddity discrimination task ▫Selectively targets perception and not memory

Buckley et al 2001

Perirhinal Lesions in Monkeys Buckley 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 Monkeys Buckley 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 Monkeys Buckley 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 Monkeys Bussey et al (2003) 1 st task: Monkeys performed a simple visual discrimination task 2 nd 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 Monkeys Baxter (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 Monkeys CONCLUSIONS 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 Humans Stark & 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 Humans Lee et al (2005c) Tested MTL-damaged and hippocampal-damaged patients on difficult visual discriminations

Lee et al 2005c

Perirhinal Lesions in Humans Lee et al (2005c)

Perirhinal Lesions in Humans Shrager et al (2006) Replicated both tasks by Lee and colleagues (2005c)

Perirhinal Lesions in Humans Shrager 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 Humans Conflicting 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 Humans Conflicting 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?