Figure 11. Effects of fixed delay pairing.

Conventions are as in Fig. 3, except for the fact that the visual stimulation has been synchronized with the ‘off’ transition of the stimulus. A, de novo appearance of a late ‘off’ response in a simple cell recorded in a 15-week-old kitten. The paired position (left column), located at the border between ‘on’ and ‘off’ fields, initially exhibited a mixed ‘on-off’ response. During pairing, a positive current pulse (S⁺, +5 nA) was applied systematically 1 s before the onset of the stimulus, and resulted in a significant imposed tonic level of activity during the late part of the ‘off’ period. The early ‘off’ response was paired with a pulse of negative current (S⁻, -5 nA) applied 1 s before the extinction of the stimulus, and its amplitude was reduced significantly (as shown in P). When recording continued without any current (+10 min), the neuron fired exactly 1 s before the effective onset of the light bar in the paired position, reproducing the exact temporal pattern of response that had been imposed during pairing. No significant changes in ‘on-off’ responses were observed in the unpaired position (right column), suggesting that the effect of the pairing was associative and restricted to the inputs during conditioning. Calibration bars: 1 s; 10 AP s⁻¹. B, lagged ‘off’ responses (latency > 1 s) were occasionally observed during control recordings in young kittens (here 6 weeks old). The latency of this form of late response is not the result of a global synchronization effect of neuronal activity with the rate of stimulation, since it was not affected when the duration of the ‘off’ period was increased from 2 to 7 s. A strong inhibition of this response was induced when the stimulus was again turned ‘on’, an observation which is similar to that also observed in A and C. Calibration bars: 1 s; 10 AP s⁻¹. C, simple cell recorded in a 17-week-old kitten. Two positions, chosen in the centre of the ‘on’ field (left column) and ‘off’ fields (right column) were unpaired, whereas a more intermediate position between the 2 fields was selected as the position to be stimulated during pairing (P). A +4 nA current pulse (S⁺) was applied 1 s before the ‘on’ transition of the stimulus, which resulted in a tonic activation of the cell (similar to that shown in A). A negative -5 nA pulse (S⁻) was also used to block the late ‘on’ activity of the cell and reduced the amplitude of the early ‘off’ response of the cell. This differential FDP was repeated 50 times. Fifteen minutes after pairing, the temporal activity pattern observed in the paired position in the absence of iontophoretic current still mimicked that imposed during pairing, whereas the ‘on’ and ‘off’ responses in the unpaired regions had similar time courses to those observed before pairing. Calibration bars: 1 s; 20 AP s⁻¹.