. Microelectrode studies in nonhuman primates and other mammals have demonstrated that many neurons in auditory cortex are excited by pure tone stimulation only when the tone's frequency lies within a narrow range of the audible spectrum. However, the effects of auditory cortex lesions in animals and humans have been interpreted as evidence against the notion that neuronal frequency selectivity is functionally relevant to frequency discrimination. Here we report psychophysical and anatomical evidence in favor of the hypothesis that fine-grained frequency resolution at the perceptual level relies on neuronal frequency selectivity in auditory cortex. An adaptive procedure was used to measure difference thresholds for pure tone frequency discrimination in five humans with focal brain lesions and eight normal controls. Only the patient with bilateral lesions of primary auditory cortex and surrounding areas showed markedly elevated frequency difference thresholds: Weber fractions for frequency direction discrimination ("higher"-"lower" pitch judgments) were about eightfold higher than Weber fractions measured in patients with unilateral lesions of auditory cortex, auditory midbrain, or dorsolateral frontal cortex; Weber fractions for frequency change discrimination ("same"-"different" pitch judgments) were about seven times higher. In contrast, pure-tone detection thresholds, difference thresholds for pure tone duration discrimination centered at 500 ms, difference thresholds for vibrotactile intensity discrimination, and judgments of visual line orientation were within normal limits or only mildly impaired following bilateral auditory cortex lesions. In light of current knowledge about the physiology and anatomy of primate auditory cortex and a review of previous lesion studies, we interpret the present results as evidence that fine-grained frequency processing at the perceptual level relies on the integrity of finely tuned neurons in auditory cortex. I N T R O D U C T I O NMany neurons in mammalian auditory cortex respond selectively to pure tone frequencies over a narrow range of the audible spectrum, and several anatomical subdivisions of auditory cortex are organized topographically with respect to frequency selectivity (for reviews, see Aitkin et al. 1984;Brugge and Reale 1985;Clarey et al. 1992;de Ribaupierre 1997;Kaas et al. 1999;Phillips et al. 1991;Schreiner 1992). On theoretical grounds, it would seem reasonable to propose that neuronal frequency selectivity at the physiological level contributes to frequency discrimination at the perceptual level. However, the functional effects of auditory cortex lesions in animals and humans have been interpreted as evidence against this hypothesis. In their authoritative review of a large number of lesion studies in the Handbook of Sensory Physiology, Neff et al. (1975) concluded, "Frequency discrimination may be learned or relearned after bilateral lesions involving all or nearly all of primary auditory cortex in animals such as the cat and monkey and in human patient...

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