Information properties of transient responses to motion in visual cortex area MT

Simon Schultz


Neurons in area MT of the monkey visual cortex often exhibit distinct transient responses to step changes in visual motion, in addition to a sustained response. We were interested in understanding whether spikes in this initial transient had qualitatively or quantitatively different information properties to those from the sustained response. We recorded extracellularly from single units in the anaesthetised, paralysed macaque monkey during presentation of brief (320 ms) visual motion stimuli consisting of drifting gratings and plaids. The reliability of spike counts in the transient was slightly greater than in the sustained response, as measured by a signficantly lower Fano factor. It appears that this effect can be accounted for by relative refractory effects at elevated firing rates. The information efficiency of the transient spikes (the spike count mutual information about motion direction normalised by the response entropy used to convey it) was however no higher than for sustained spiking activity. The transient response did however show differences from the sustained in the temporal information content of the spike trains, with significant synergy and redundancy between spikes observed only in the transient. This synergy/redundancy was due to stimulus-independent spike time correlations, was distributed about zero but with a wide spread between cells, and may be due to characteristic but brief temporal structure in onset transients for individual cells.