Estimation of Local and Global Characteristics of Half-Tone Surfaces by the Cat's Lateral Geniculate Body

Abstract

We investigated neuronal mechanisms in the cat's dorsal LGN that could carry information about the spatial brightness distribution across half-tone visual object surfaces. Stimuli were halftone spheres, cylinders, toroids, etc. We have found that geniculate neuronal structures describe and estimate both local and global characteristics of the brightness distribution on these surfaces. The search for local mechanisms of perception of half-tone surfaces revealed a new property of geniculate neurons: their sensitivity to changes in both the magnitude and vector direction of the spatial brightness gradient in the test stimulus. The quantitative estimation of gradient sensitivity in each case has been calculated by the formula: K=(N_max-N_min)/N_max, where N_max is the number of spikes (in a 500 ms interval) in response to the preferred (and N_min to the nonpreferred) gradient direction. The average value of K was 0.49 (range 0.15 – 0.95) for a mean brightness gradient of 2.7 cd m⁻² deg⁻¹. A ninefold decrease of the gradient reduced K by a factor of 6.5. The preferred direction of most (82%) neurons coincided with the radial direction from the centre of the receptive field to area centralis. The basis of this phenomenon probably lies in the influence of the spatial brightness gradient on spatiotemporal relations between excitatory and inhibitory processes in geniculate receptive fields. Global characteristics of the surface brightness distribution were represented in the population response of the neurons. The pattern of neuronal activity elicited in on-neurons and off-neurons in response to surface stimuli correlated with the isophot structure of the surface (r_av=0.86 for 23 cases). Therefore, separate descriptions of local and global brightness distributions originate at subcortical levels of the visual system.