ELSEVIER Hearing Research 72 (1994) 237-253 In search of the best stimulus: An optimization procedure for finding efficient stimuli in the cat auditory cortex I. Nelken *, Y. Prut, E. Vaadia, M. Abeles zyxwvutsrqponmlkjihgfedcbaZYXW Department of Physiology, The Hadassah Medical School, .kru.salm~ 91010, l,vruel (Received 7 October 1992; Revision received 1 September 1993; Accepted 15 September 19Y3) Abstract Units in the auditory cortex of cats respond to a large variety of stimuli: pure tones, AM- and FM-modulated signals, clicks, wideband noise, natural sounds, and more. However, no single family of sounds was found to be optimal (in the sense that oriented lines are optimal in the visual cortex). The search for optimal complex sounds is hard because of the high dimensionality of the space of interesting sounds. In an effort to overcome this problem, an automatic search procedure for finding efficient stimuli in high-dimensional sound spaces was developed. This procedure chooses the stimuli to be presented according to the responses to past stimuli, trying to increase the strength of the response. The results of applying this method to recordings of population activity in the primary auditory cortex of cats are described. The search was applied to single tones. two-tone stimuli, four-tone stimuli and to a two-dimensional subset of nine-tone stimuli, parametrized by the center frequency and the fixed difference between adjacent frequencies. The method was able to find efficient stimuli, and its performance improved with the dimension of the sound spaces. Efficient stimuli, found in different optimization runs using population activity recorded from the same electrode, often shared similar frequencies and pairs of frequencies, and tended to evoke similar levels of activity. This result indicates that a global analysis of the location of spectral peaks is performed at the level of the auditory cortex. Key words: Primary auditory cortex; Complex sounds; Optimal stimuli; Cat 1. Introduction One common view of the way the cortex processes sensory stimuli is that each neuron or local group of neurons have an adequate stimulus, which has two properties: first, it evokes a stronger response (in terms of firing rate) than other stimuli, and second, the parameters of this adequate stimulus are mapped in an orderly way on the surface of the cortex. Such an organization was described in the primary visual cortex, where orientation selectivity and eye dominance are accepted as the main parameters of the adequate stim- ulus (Hubel and Wiesel, 1977). In particular, orienta- tion selectivity does not occur at lower stations of the visual pathways, and is presumably a result of cortical processing of the visual inputs. However, in the pri- * Corresponding author. The Johns Hopkins School of Medicine, Center for Hearing Sciences, 720 Rutland Avenue, Baltimore, MD 21205. USA 0378.5955/04/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDl 0378.5~~55(93)EOlS8-8 mary auditory cortex and also in higher stations of the visual system (e.g. field IT, Oram and Perrett, 1992) the situation is less clear. The primary auditory cortex lies in the middle of the auditory pathways, between the subcortical stations and the higher, secondary and associative cortices (Aitkin, 1990; Brugge and Reale, 1985; Schreiner and Merzenich, 1988). The flow of auditory information, which starts with the spectral analysis at the cochlea. undergoes major transformations on its way to the cortex, most of which are not well understood (Aitkin, 1990; Sachs and Blackburn, 1991; Ehret and Merzenich. 1988; Yeshurun et al., 1985). Two types of organizations have been described at the level of the auditory cortex, the tonotopic gradient and the binaural interaction bands. The cortical tono- topic order (Goldstein et al., 1970; Goldstein and Abeles, 1975; Merzenich et al., 1975) probably mirrors the tonotopic order which is kept along the core audi- tory pathway starting at the auditory nerve (Aitkin, 1990; Schreiner and Merzenich, 1988). The binaural