Cortical circuits mediating direction and orientation selective
responses and responses from outside the classical receptive field
in visual cortex.
Single and multiple spike train analysis, crosscorrelation techniques,
intracellular recording in vivo.
Studies are on-going in two, related major areas: extracellular
studies of the context dependency of responses elicited from the
classical receptive fields (CRF) of neurons in cat primary visual
cortex and intracellular studies of lateral, horizontal inputs
to these same cells.
We have found that the responses of about half of the direction
selective cells in primary visual cortex can be strongly influenced,
sometimes even overwhelmed, by moving stimuli situated entirely
outside the CRF. The centers and surrounds have an opponent organization--surround
motion in the null direction facilitates the center response and
surround motion in the preferred direction inhibits the center
response. By estimating spatiotemporal receptive field structure
both with and without effective surround stimuli, we have determined
that the surround effects are effected by changes in gain of the
CRF mechanism rather than changes in RF structure. Thus the filters
are scalable but not tunable.
In the intracellular studies, we are using spike-triggered averaging
of membrane potential locked to extracellular activity of groups
of neurons recorded extracellularly at various distances in visual
space and as a function of orientation difference. With this approach,
we hope to establish patterns of connectivity and to explore the
synaptic mechanisms underlying context dependency in visual cortex.
KEY WORDS: visual cortex, context dependency, extracellular
recording, intracellular recording.
Palmer, LA and McLean, J. 1999. Motion effects
from the surround in visual cortical receptive fields. Investig.
Ophthal. And Vis. Science, 40:201.
Wolfe, J. and L.A. Palmer. 1998. Temporal diversity in the lateral
geniculate nucleus of the cat. Visual Neuroscience, 15:653-675.
McLean, J, Raab, S. and L.A.Palmer. 1994 Contributions of linear
mechanisms to the specification of local motion by simple cells
in areas 17 and 18 of the cat. Visual Neuroscience, 11:271-294.
