Abstract
We investigated how physiologically observed forward suppression interacts with stimulus frequency in neuronal responses in the guinea pig auditory cortex. The temporal order and frequency proximity of sounds influence both their perception and neuronal responses. Psychophysically, preceding sounds (conditioners) can make successive sounds (probes) harder to hear. These effects are larger when the two sounds are spectrally similar. Physiological forward suppression is usually maximal for conditioner tones near to a unit's characteristic frequency (CF), the frequency to which a neuron is most sensitive. However, in most physiological studies, the frequency of the probe tone and CF are identical, so the role of unit CF and probe frequency cannot be distinguished. Here, we systemically varied the frequency of the probe tone, and found that the tuning of suppression was often more closely related to the frequency of the probe tone than to the unit's CF, i.e. suppressed tuning was specific to probe frequency. This relationship was maintained for all measured gaps between the conditioner and the probe tones. However, when the probe frequency and CF were similar, CF tended to determine suppressed tuning. In addition, the bandwidth of suppression was slightly wider for off-CF probes. Changes in tuning were also reflected in the firing rate in response to probe tones, which was maximally reduced when probe and conditioner tones were matched in frequency. These data are consistent with the idea that cortical neurons receive convergent inputs with a wide range of tuning properties that can adapt independently.
Highlights
The responses of auditory neurons decrease over time with continuous or repeated stimulation
When the probe frequency was 20 kHz, near to the unit characteristic frequency (CF), the suppressed receptive field (RF) (SRF) had a similar shape to the RF, and both the suppressed CF (SCF) and suppressed BF (SBF) are at the probe tone frequency
When the probe tone frequency was at 12 kHz, well below the unit’s CF, the SRF shifted to lower frequencies
Summary
The responses of auditory neurons decrease over time with continuous or repeated stimulation. In cortical neurons, the tuning of the forward suppression is often qualitatively different from the excitatory RF and not well accounted for by preceding spiking activity (Calford & Semple, 1995; Brosch & Schreiner, 1997). Previous studies of forward suppression using two tones have concentrated on the case when the probe tone is at the characteristic frequency (CF) of the neuron (Harris & Dallos, 1979; Boettcher et al, 1990; Brosch & Schreiner, 1997). In the inferior colliculus (IC) there is some evidence that setting the probe frequency away from CF affects the tuning of suppression (Malone & Semple, 2001) This may depend on the location within the central nucleus of the IC (Stakhovskaya et al, 2008). As it remains unclear whether forward suppression in the cortex is determined by the probe frequency or the tuning properties of the neuron, we investigated systematically the extent to which forward suppression in cortical neurons is specific to the frequency of the probe in a two-tone sequence
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