Signal detection theory yes no Visual stimulus Neurometric function Psychometric function Neuronal response? Choice probability Behavioral judgement Direct comparison oerception and neural activity Record neuron that prefers downward motion while monkeys make up/down motion judgments for stimuli that are barely discriminable. Newsome, Movshon, Britten, Shadlen, Salzman
Visual area MT Record 7a LIP, VIP V1 DP M V4 T MST FEF Visual input V 2 VP VOT FST PITd S TP PITv CITd CITv AITd AITv Visual area MT MST MT V3a V3 V2 V1 Electrophysiology (action potential) microelectrode amplifier Voltage (mv) Time (msec) Cortical pyramidal cell (Golgi stain)
MT neurons are direction-selective MT neurons are direction-selective Maunsell and Van Essen, 1983 Behavioral protocol Dots Aperture Receptive field Pref target Null target 10 deg Fixation Point Fix Pt Dots Targets 1 sec
Stimulus manipulation: motion coherence 0% coherence 50% coherence 100% coherence Psychometric function Britten, Shadlen, Newsome & Movshon, 1992 Motion coherence and MT neurons Motion stimulus no coherence 50% coherence 100% coherence Responses of MT neurons Preferred direction
Motion coherence and MT neurons Neural responses are noisy Each tick is an action potential Each line corresponds to a stimulus presentation Average across all trials Firing rate (sp/sec) Trial number Time (msec) Perceptual decision Decision rule: choose pref iref response > non-pref response. Probability r Neuronal response : response PDF for pref direction : response PDF for non-pref direction
Perceptual decision Probability r p : response to pref direction r n : response to non-pref direction : response PDF for pref direction : response PDF for non-pref direction F n : response CDF for non-pref direction 0 r r Neuronal response P(correct) = P(r p > r n ) = r ( r )d r 0 dr ( r )d r = F n P(correct) = F n dr 0 0 Neurometric function Response distributions P(correct) = r [r]f n [r] Neurometric function 6.4% Response (spikes/trial) I 1 1 I I,'"I 1.o 10.0 100 Correlation (%) llustrates frequency histograms of responses obtained from a single Britten, Shadlen, Newsome & Movshon, 1992 Neurometric vs. psychometric functions psychometric neurometric Neuronal threshold (%) 1 10 100 1 10 100 Correlation (%) Figure 6. Psychometric and neurometric functions obtained in six Britten, Shadlen, Newsome & Movshon, 1992
Visual stimulus Neurometric function Psychometric function Neuronal response? Choice probability Behavioral judgement Predicting the monkey s decisions Neurometric & psychometric functions: accuracy vs motion coherence Response distributions for pref and non-pref decisions at a fixed motion coherence 1 0.9 50 45 0% motion coherence Proportion correct 0.8 0.7 0.6 0.5 Neuron Behavior 0.4 0.1 1 10 100 Response (impulses/sec) 40 35 30 25 20 15 10 5 0 Correct pref decisions trials Error non-pref trials decisions 10 20 30 40 50 60 70 80 Motion strength (% coherence) Trial number Shadlen, Britten, Newsome & Movshon, 1996 Predicting the monkey s decisions Choice probability: accuracy with which one could predict monkey s decision from the response of the neuron given that you know the distributions. f N f P Probability Neuronal response f P : response PDF when monkey reports pref direction f N : response PDF when monkey reports non-pref direction
Choice probability Example neuron Across all neurons recorded Britten, Newsome, Shadlen, Celebrini & Movshon, 1996 Computational model Pooled "up" signal MT neuron pool + Σ Pooling noise Decision + Pooling noise Pooled "down" signal Noise is partially correlated across neurons. Responses are pooled non-optimally over large populations of neurons including those that are not the most selective. Additional noise is added after pooling. Shadlen, Britten, Newsome & Movshon, 1996 Damage to MT affects performance Newsome and Paré, 1988
Microstimulation in MT changes motion perception Salzman, Britten, Newsome, 1990