M-cell responses to tectal stimulation. A. M-cell responses to a 60 Hz tectal train of 200 ms were used to measure the evoked depolarization at 1, 33, 100 and 200 ms after stimulus onset during a 12-ms time window (colored vertical boxes). Á. Lower inset. Black and grey bars in the trace show amplitudes of the tonic and phasic components of the response, respectively. The horizontal dotted line indicates the baseline for quantification of each component. B. Mean amplitude (± SEM) of the phasic (shaded) and tonic (solid) components of tectal trains for the time windows described in A. C. M-cell responses to a 100 ms tectal pulse train of 30, 60, 100 or 200 Hz were recorded and quantified as in A, at the end of the pulse. D. Mean amplitude (± SEM) of the phasic (shaded) and tonic (solid) components of 100-ms tectal trains of different frequency.

Multisensory integration in the M-cell. A. Diagram of stimulation (left) and response examples (right). The response examples correspond, to a single acoustic pulse (blue trace) and to their multisensory combination (purple). Below the traces diagram of the stimulation is shown in gray. B. Stacked bars comparing mean (± SEM) depolarization evoked by tectal trains of indicated duration (T, orange), auditory (A, blue) or their multisensory combination (purple). C. Boxplots showing the ratios between the multisensory responses and the maximum unisensory response (MSI/Max). D. Boxplots showing the ratios between the multisensory responses and the sum of its unisensory components (MSI/Sum). E-H. As in A-D but for 100 ms tectal trains of different frequencies.

Auditory and tectal FFI have different decay dynamics. A. Diagram of auditory or tectal feedforward inhibition circuits (center), and the quantification of sensory evoked shunting inhibition as a reduction of M-cell AP amplitude (left, auditory: shaded-blue area; right, tectal: shaded-red area) to a control AP (black trace). B-C. Boxplots of peak inhibition and time of peak inhibition triggered by acoustic or tectal stimuli. D. Mean (±SEM) FFI triggered by tectal and auditory inputs measured in the same M-cells at different time points between 0 and 70 ms post-stimulus. E. Boxplot of the time elapsed to 50% decay of peak FFI for the auditory (left) and tectal (right) stimuli. *p=0.0145. F. Area below the curve (0-70 ms) for the auditory (left) and tectal (right) stimuli. *p=0.0181.

Effect of stimulus sequence and modality on M-cell integration. A. Representative responses of one M-cell to two stimuli presented either in different trials, or in the same trials with a 50 ms offset. Schematics below each trace the order of stimuli and the area where responses were quantified. The pink bar indicates the time point at which the lingering depolarization of the first response was measured. Note that this analysis was performed for trials in which Stim1 and Stim2 could be either A or T, resulting in trials AA, TT, TA and AT sequences. B. Lingering depolarization after 50 ms of a single auditory pip or a short tectal train. *p=0.0188. C. Integration in the M-cell for pairs of uni- or multisensory stimuli with a 50 ms delay was calculated as the ratio: Resp (1+2) / (Resp1 + Resp2).

Multisensory integration of subthreshold unisensory stimuli. A. MSI/Max vs. the maximum unisensory response of different multisensory stimuli, irrespective of duration, frequency, or sequence of stimulation. B. Similar to A, but with the MSI/Sum instead vs. the sum of the unisensory responses. C. Example traces in which the integration of subthreshold unisensory tectal (T, red) and auditory (A, blue) stimuli evokes an M-cell AP (TA, purple). Responses to an auditory pip only triggered an action potential when they were preceded by four tectal pulses delivered at 333Hz (left) or a 200-ms tectal train at 60Hz (right).

A-B. Representative recordings of left or right M-cell somatic responses to an electric stimulus train delivered to the left tectum (A) or to a short 200 Hz sound pip produced by a loudspeaker placed to the right side of the fish (B). The mean (of 3-5 repetitions) response is shown in solid color and the diagram of the tectal stimulus or the the sound pip are shown below the traces. Dashed boxes indicate the 12-ms time windows following the stimulus where the responses were quantified. C-D. Boxplots showing the median, upper and lower quartiles and comparing the response to the same stimulus in the left or right M-cell in 9 fish for the tectal stimulus (C) and 11 fish for the auditory stimulus (D). Markers with the same color in left and right belong to pairs of M-cells of the same fish. No differences were found in the evoked response amplitude between the left and right M-cells or for tectal or auditory stimuli.