BRAIN-STEM MODULATION OF SIGNAL TRANSMISSION THROUGH THE CAT DORSAL LATERAL GENICULATE-NUCLEUS

We studied changes in retinogeniculate transmission that occur during variation of modulatory brainstem input and during Variation of stimul us contrast. Responses of single cells in the dorsal lateral geniculat e nucleus (dLGN) to a stationary flashing light spot of varying contra st were measured with and without electrical stimulation of the peribr achial region (PER) of the brainstem. PER stimulation increased the co ntrast gain (slope of response versus contrast curve) and the dynamic response range (range between spontaneous activity and maximal firing) . Lagged and nonlagged X-cells reached the midpoint of the dynamic res ponse range at lower contrasts during PER stimulation than ih the cont rols. No comparable change was seen for Y-cells. Only minor changes of threshold contrast were seen. The characteristics of the retinogenicu late transmission were directly studied by comparing the response of d LGN cells with their retinal input (slow potentials, S-potentials). Wi th increasing contrast there was a marked increase in the transfer rat io (proportion of impulses in the input that generates action potentia ls in the dLGN cell). The transfer ratio seemed to be primarily determ ined by the firing rate of the retinal input. The transfer ratio incre ased with increasing input rates from low values near threshold to val ues that could approach 1 at high-input firing rates. PER stimulation increased the transfer ratio, particularly at moderate input firing ra tes. The increased transfer ratio, caused by increasing input firing r ates, enhanced the response versus contrast characteristics through an increase in contrast gain and dynamic response range. The modulatory input from the PER further enhanced these characteristics.