Fig. 2.

Inactivation of an MT direction column can evoke clear and reversible changes in the responses of LGN cells. (A, B) The histograms document the responses of two typical LGN cell examples to a large patch of grating drifting in the preferred (left) and non-preferred (right) directions of the MT inactivation site, before (C), during (G) and after cessation of (R) GABA application in MT. Red shading indicates that there was a significant difference (P < 0.05, paired t-test, see Experimental procedures) between the responses observed during control and MT inactivation conditions. Error bars 1 SE. Scale bars = 10 s/s. Here, and in all subsequent panels and figures, the iconic representation of the MT and LGN cell receptive field locations and stimulus configuration drawn above the records are for illustrative purposes only, and not to scale. (C, D) The histograms document the responses of two typical LGN cell examples to a large patch of texture drifting in the preferred (left) and non-preferred (right) directions of the MT inactivation sites before (C), during (G) and after cessation of (R) iontophoretic application of GABA in MT. Error bars 1 SE. Scale bars = 20 s/s. (E, F) Repeatability of influence of MT inactivation over time. The histograms document the responses of an LGN cell to a patch of grating drifting in the preferred (left) and opposite directions of motion (right) of the MT inactivation site, before (C), during (G) and after cessation of (R) GABA application in MT. The histogram in (E) shows the responses during the initial application of GABA in MT and that in (F) the responses observed during a subsequent test cycle recorded over an hour later. Scale bar = 10 s/s. (G, H) Reciprocal effects of inactivation of an MT direction column on LGN cell responses to the MT site preferred direction of motion and its reverse. The histograms document the responses of two LGN cells to a patch of grating drifting in the preferred (left) and opposite directions of motion (right) of the MT inactivation site, before (C), during (G) and after cessation of (R) GABA application in MT. Scale bar = 15 s/s. (I–K) Iontophoretic application of GABA and the GABA B receptor antagonist CGP 55845 in MT evoke opposite effects on the responses of LGN cells. The histograms in (I) document the responses of an LGN cell to a patch of grating drifting in the preferred (left) and opposite directions of motion (right) of the MT locus, before (C), during (CGP) and after cessation of (R) CGP 55845 application in MT. The histograms in (J) shows the responses of the same cell observed during GABA application in MT. Scale bar = 15 s/s. During CGP application in MT, the LGN cell’s response to the stimulus moving in the MT preferred direction of motion was significantly reduced whereas it was significantly increased during GABA iontophoresis in MT. This reciprocal pattern of effect held across our sample (n = 5) of cells tested with both GABA and CGP 55845, with a significant negative correlation in the normalized response change observed in the two conditions (R = −0.9, P = 0.037, Spearman rank order correlation test). The polar plots in (K) document the responses (in s/s) of the MT locus to a large patch of drifting grating presented at a range of test directions before (black) and during (blue) local micro-iontophoretic application of CGP 55845. There was no response during local iontophoretic application of GABA (green). Scale bar = 80 s/s. (L–O) Differential effects of inactivation of an MT direction column on LGN cell responses to single and paired stimulus configurations. The histograms in (L) document the responses of an LGN cell to a large, single patch of grating drifting in the preferred (left) and opposite directions of motion of the MT inactivation site (right), before (C), during (G) and after cessation of (R) GABA application in MT. Scale bar = 15 s/s. The histograms in (M) document the responses of the same LGN cell in the presence of two patches of grating, located over the MT and LGN cell receptive fields respectively before (C), during (G) and after cessation of (R) GABA application in MT. Both patches drifted either in the preferred (left) or opposite (right) directions of motion of the MT inactivation site. Scale bar = 15 s/s. The histograms in (N, O) document the response of another LGN cell, before (C), during (G) and after cessation of (R) GABA application in MT to a large, single patch of grating (N) and to two patches of grating (O) drifting in the preferred and non-preferred directions of motion of the MT inactivation locus. Scale bars = 15 s/s. (P, Q). Inactivation of an MT direction column evokes clear and reversible changes in LGN cell responses to flashing stimuli. The histograms document the responses of two LGN cells before (C), during (G) and after cessation of (R) GABA application in MT. The stimulus located over the LGN cell RF comprised a stationary flashing spot while the stimulus located over the MT cell RF comprised a patch of grating, drifting in either the preferred (left) or non-preferred (right) directions of motion of the MT inactivation site. Scale bars = 10 s/s. Error bars 1 SE.