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