Figure 1.

Extracellularly recorded responses of a single γ-type neuron to five different acoustic waveforms. A noisy membrane current was added to every neuron in the simulation of the neuronal mathematics for the organism, to simulate the noise caused by other inputs that would always be present in a real biological system. Before the experiment, the network parameters were set by using only a single exemplar of “one” spoken by speaker a, plus single examples of nine other different patterns (each recognized by one of nine other γ-neurons, not shown here). (a) Spike rasters, aligned in time to the start of the acoustic waveform shown in the Inset, in response to eight different trials using an utterance of “one” from speaker a (not the training exemplar). Below the rasters is their corresponding peristimulus time histogram (PSTH), smoothed by a Gaussian with a standard deviation of 12 ms. The γ-cell begins spiking near the end of the word. Tick marks in the Inset correspond to 0 and 500 ms. (b) Same format as a, for an utterance of the word “one” from a different speaker (speaker b). (c) Same format as a for a “one” spoken by speaker a in the presence of a loud tone at 800 Hz. The waveforms are markedly different in a, b, and c, but the γ-cell responds to all. (d) Same format and utterance as in a, but the acoustic waveform has been reversed in time. (e) Same format as a, for an utterance by speaker b of the word “three.” Few or no spikes occurred in response to the waveforms of d and e. Other, similar-sounding words (for example, “wonder”) occasionally cause the cell to fire as well, indicating that these output cells are not completely specific but merely encode utterances quite sparsely.