Estimating Rats’ Working Memory Estimating Rats’ Working Memory Capacity in an Object Recognition Capacity in an Object Recognition

Foraging TaskForaging Task

Jerome Cohen, Xue Han, Anca Matei, Vara Jerome Cohen, Xue Han, Anca Matei, Vara ParameswaranParameswaran

Department of Psychology, University of WindsorDepartment of Psychology, University of WindsorMyron HlynkaMyron Hlynka

Department of Mathematics and Statistics, University of Department of Mathematics and Statistics, University of WindsorWindsor

The14th Annual Meeting of the Comparative Cognition Society

Melbourne Beach, Florida, March, 2007

Difficulties in assessing memory in Difficulties in assessing memory in non-verbal animalsnon-verbal animals

DMTS, DNMTS, radial maze, food cache DMTS, DNMTS, radial maze, food cache recovery errors not always due to loss in working recovery errors not always due to loss in working memory (Thorpe, Jacova, Wilkie, 2004; Wilkie, memory (Thorpe, Jacova, Wilkie, 2004; Wilkie, Willson, Carr, 1999)Willson, Carr, 1999)

Overestimation (Cole & Chappell-Stephenson, Overestimation (Cole & Chappell-Stephenson, 2004) or underestimation (Cohen et al. 2004) of 2004) or underestimation (Cohen et al. 2004) of spatial location working memory in radial maze spatial location working memory in radial maze tasktask

How much information can be How much information can be stored and for how long?stored and for how long?

Open radial maze tasks confounded with Open radial maze tasks confounded with direction cuesdirection cues Rats may retain how far away arms are from a given Rats may retain how far away arms are from a given

direction sampled rather than specific arm locations. direction sampled rather than specific arm locations.

Object recognition tasks: DMTS, DNMTS, novel Object recognition tasks: DMTS, DNMTS, novel object recognition (see Mumby, 2001)object recognition (see Mumby, 2001) Assess retention interval for one or two objectsAssess retention interval for one or two objects Non-spatial DMTS, DNMTS versions difficult to Non-spatial DMTS, DNMTS versions difficult to

acquire. acquire.

Five important characteristics of a good Five important characteristics of a good working memory task for ratsworking memory task for rats

Object recognition task should be easy to acquireObject recognition task should be easy to acquire

Working memory should be biologically more relevant to Working memory should be biologically more relevant to the animal than search response algorithmsthe animal than search response algorithms

Reward new object recovery so as to prevent premature Reward new object recovery so as to prevent premature search terminationsearch termination

Large pool of objects to prevent proactive interference Large pool of objects to prevent proactive interference from object repetition over successive trialsfrom object repetition over successive trials

Assess individual animal’s working memory capacityAssess individual animal’s working memory capacity

Our Object Location Recognition Task Our Object Location Recognition Task ((version 2 of Cohen et al.2006)version 2 of Cohen et al.2006)

Subjects: Six Long-Evans male hooded ratsSubjects: Six Long-Evans male hooded rats

Apparatus: Large enclosed square foraging area with 25 (5 x 5) Apparatus: Large enclosed square foraging area with 25 (5 x 5) covered food wellscovered food wells Only food wells covered with objects baited with sunflower Only food wells covered with objects baited with sunflower

seedsseeds Pool of 60 objectsPool of 60 objects

Trials partitioned into Trials partitioned into studystudy and and testtest segments segments StudyStudy segment: n object-covered wells, each baited with one segment: n object-covered wells, each baited with one

seedseed TestTest segment: one or more ‘new’ objects replace ‘old’ objects - segment: one or more ‘new’ objects replace ‘old’ objects -

baited with 6 seedsbaited with 6 seeds I-min inter-segment-intervalI-min inter-segment-interval

ThreeThree PhasesPhases – – each 48 trialseach 48 trials

Phase 1 trial: One Phase 1 trial: One new new object in object in test test segment segment replaces one of the replaces one of the threethree old old objects from objects from previous previous study study segment. segment. Object locations randomized over trials but Object locations randomized over trials but

not within a trialnot within a trial

Phase 2 trial: Two Phase 2 trial: Two newnew objects in objects in testtest segment replace two of the segment replace two of the sixsix old old objects objects from previous from previous study study segmentsegment

Phase 3 trial: Three Phase 3 trial: Three newnew objects in objects in testtest segment replaces three of segment replaces three of ninenine old old objects objects from previous from previous study study segment.segment.

Dependent Variable and Measure of Estimated Dependent Variable and Measure of Estimated No. of ‘Old’ Objects RetainedNo. of ‘Old’ Objects Retained

Number of choices to find all “jackpot” objectsNumber of choices to find all “jackpot” objects

Observed distribution of cumulative proportion of Observed distribution of cumulative proportion of trials rat finds all jackpots in each phase.trials rat finds all jackpots in each phase.

Compare observed distributions with theoretical Compare observed distributions with theoretical cumulative probability distributions for finding all cumulative probability distributions for finding all jackpots based on no memory (chance) to perfect jackpots based on no memory (chance) to perfect working memory of all ‘old’ objects from the study working memory of all ‘old’ objects from the study segment (K-S tests)segment (K-S tests)

Estimate of recognized old objects based on 95% Estimate of recognized old objects based on 95% confidence interval around the observed distribution.confidence interval around the observed distribution.

Example for one rat

Summary of ResultsSummary of ResultsPhase 1: No rat shows above chance Phase 1: No rat shows above chance performance in finding one jackpot out of three performance in finding one jackpot out of three objects. objects.

Phase 2: All rats show above chance Phase 2: All rats show above chance performance in finding two jackpots out of six performance in finding two jackpots out of six objects. objects.

Phase 3: Five rats show above chance Phase 3: Five rats show above chance performance in finding three jackpots out of nine performance in finding three jackpots out of nine objects.objects.

Estimation of Number of Old Estimation of Number of Old Objects Recognized Objects Recognized

Phase 2: Six objects / Phase 3: Nine objectsPhase 2: Six objects / Phase 3: Nine objects Rat 2B between 1 and 4 / 0 and 4 Rat 2B between 1 and 4 / 0 and 4 Rat 2D between 2 and 4 / 6 and 8Rat 2D between 2 and 4 / 6 and 8 Rat 3A between 0 and 3 / noneRat 3A between 0 and 3 / none Rat 3B between 1 and 4 / 2 and 4Rat 3B between 1 and 4 / 2 and 4 Rat 3C between 0 and 3 / 0 and 3Rat 3C between 0 and 3 / 0 and 3 Rat 3D between 0 and 3 / 0 and 3 Rat 3D between 0 and 3 / 0 and 3

Conclusions Conclusions

Increasing set or patch size promotes Increasing set or patch size promotes working memory processes relative to working memory processes relative to other search strategies.other search strategies.

Our rats seem to be able to recognize Our rats seem to be able to recognize about 2 or 3 old objects.about 2 or 3 old objects.

Does recognition of old objects Does recognition of old objects change as more jackpots are change as more jackpots are found?found?

Compare observed distributions for Compare observed distributions for finding all three jackpots with those for finding all three jackpots with those for finding two jackpots or only one jackpot finding two jackpots or only one jackpot from nine objects. from nine objects.

Generate new theoretical distributions for Generate new theoretical distributions for number of choices to find the first, second, number of choices to find the first, second, or third jackpot based on no memory to or third jackpot based on no memory to perfect memory for old objects.perfect memory for old objects.

Example for one rat

Re-estimation of Number of Re-estimation of Number of Recognized Old ObjectsRecognized Old Objects

Phase 3: 1st / 2Phase 3: 1st / 2ndnd / 3rd jackpot / 3rd jackpot 2B: 6 - 8 / 3 – 7 / 0 – 42B: 6 - 8 / 3 – 7 / 0 – 4 2D: 7 - 9 / 7 – 9 / 6 – 82D: 7 - 9 / 7 – 9 / 6 – 8 3A: 6 - 8 / 2 – 6 / none3A: 6 - 8 / 2 – 6 / none 3B: 7 - 8 / 3 – 7 / 1 - 43B: 7 - 8 / 3 – 7 / 1 - 4 3C: 5 – 8 / 2 – 6 / 0 - 33C: 5 – 8 / 2 – 6 / 0 - 3 3D: 7 - 9 / 3 – 6 / 0 - 33D: 7 - 9 / 3 – 6 / 0 - 3

Conclusion and QuestionConclusion and Question

As rat finds more jackpots, its As rat finds more jackpots, its performance for recognizing old objects performance for recognizing old objects declinesdeclines

Is this effect due to a loss in rat’s Is this effect due to a loss in rat’s working memory capacity or switching working memory capacity or switching to other search strategies?to other search strategies?

Thank You !