A Matter of Time…

Atanu Datta

Memory

TIME

Object

Space

Odor

Touch

Temporal Sequence of events objects or actions

Time and the Hippocampus

• Time cells– Found in Hippocampus with “no specific

topographic organization”– Code for time in delay intervals.– Increase or decrease their firing rate (FR) to code

for passage of time.– Track time even without active locomotion.

Previous Studies

olfactory temporal order task

CA1 hippocampal cell populations code time, by changes in population activity

Rodents

Visual temporal order task

Time cells coded fo ‘what’ items and in ‘which’ ‘sequential’ order.

Primates

AIM

To characterize incremental timing signals in object place associative learning task.

Time

ObjectSpace

Object Place Combination

O1

O2

+

+

P1

P2

•O1P1, O2P2 Early Release•O1P2, O2P1 Late Release

PARADIGM

Data Acquisition • Specific OP combination is learnt

if Pr(Correct Response) > 0.95Behavioral Training

• MRI guided insertion of microelectrodes via chamber and grid system

• Into Primate hippocampus.

Electrophysiology(n=152 recorded)

139/152 were correct trials

Results• Agnostic ITCs: n=61/139 agnostic; 16/61 ITCs

I

Increased FR n=9 Dec

reas

ed F

R n

=7

Release selective ITCs

• n=78/139, differentiated between trial types

• 43/78 were release selective early (O1P1,O2P2) vs late release (O2P1, O1P2) trials.

• n= 21/43 cells were ITCs.• n=10 FR ‘Incremental Rise’ in

firing rate• n= 11 FR ‘Incremental

Decrease’ in firing)g rate• Eg of early release increasing FR

and late release decreasing FR ITC.

OP selective ITCs• n=31/139 codes for one

particular OP combination out of 4.

• n=26/31 OP selective ITCs.• n=10/26 Incrementally

increased FR .• n=16/26 Incrementally

decreased FR.• Eg. of O2P2 selective

Decreasing FR ITC.

Learning Related Neural Signals

Percentage of ITCS recorded when at least one OP combination was learnt .

Number of OP combinations learnt in sessions in which Ag, RS, and OP ITCs were recorded.

Signals related to task performance

• Compared responses of all three categories ITCs of correct trials vs. incorrect trials.

• Zi = Z-µ (Normalization)

σZi- Z score

Z- Raw firing rateµ- Average firing rateσ- Standard DeviationZ score is a population representation of all PSTH of all trial

types.Error trial- Early release of bar in late release trial; Late release of bar in early trial.

Correct trials vs Error Trials

y-=mx+c; m =tan Ө

Release selective ITCs showed significantly greater differences in incremental tuning when comparing correct vs incorrect trials than Ag ITCs and OP ITCs.

Ag. ITC RS ITC

OP ITC

Summary• Studied delay activity of hippocampal ITCs in OP learning

task.• ITCs increased or decre3ased FR with passage of time.• Agnostic, Release Selective and OP selective ITCs were

characterized.• Release and OP selective integrated time and context.• RS and OP selective ITCs fired significantly more for learning

OP combinations than agnostic ITCs.• Thus, RS and OP ITCs are responsible for learning OP

combinations.• All three categories of ITCs showed decreased firing rate in

error trials than in correct trials.• Thus ITCs also code for task performance.

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