Responses of Neurons in Primary Auditory Cortex (A1) to Pure Tones in the Halothane-Anesthetized Cat

Abstract

The responses of primary auditory cortex (A1) neurons to pure tones in anesthetized animals are usually described as having mostly narrow, unimodal frequency tuning and phasic responses. Thus A1 neurons are believed not to carry much information about pure tones beyond sound onset. In awake cats, however, tuning may be wider and responses may have substantially longer duration. Here we analyze frequency-response areas (FRAs) and temporal-response patterns of 1,828 units in A1 of halothane-anesthetized cats. Tuning was generally wide: the total bandwidth at 40 dB above threshold was 4 octaves on average. FRA shapes were highly variable and many were diffuse, not fitting into standard classification schemes. Analyzing the temporal patterns of the largest responses of each unit revealed that only 9% of the units had pure onset responses. About 40% of the units had sustained responses throughout stimulus duration (115 ms) and 13% of the units had significant and informative responses lasting 300 ms and more after stimulus offset. We conclude that under halothane anesthesia, neural responses show many of the characteristics of awake responses. Furthermore, A1 units maintain sensory information in their activity not only throughout sound presentation but also for hundreds of milliseconds after stimulus offset, thus possibly playing a role in sensory memory.

Level Invariant Representation of Sounds by Populations of Neurons in Primary Auditory Cortex

“…In contrast to our results, previous studies in anesthetized cat (Schreiner et al, 2000;Moshitch et al, 2006) and awake primates (Recanzone et al, 2000) show that most A1 units have V-shaped FRAs. Several factors could lead to the discrepancy between the results of the present and previous studies: (1) anesthetic effects and related laminar biases, (2) coarse sampling with search stimuli, (3) search biases attributable to inherent differences in response properties between O and I/V units, and (4) potential species specificity.…”

“…Figure 1 B depicts an I unit, which exhibited the same bandwidth at all levels tested, resulting in a characteristic I-shaped FRA. Together, these two types of responses are similar to traditional descriptions of FRAs in A1 (Recanzone et al, 2000;Schreiner et al, 2000;Moshitch et al, 2006). In the present study, however, only 100 of 275 FRAs (36%) were classified as V or I shaped.…”

“…Whereas I/V units populated this two-dimensional parameter space broadly and showed some overlap, O units were more selective, as evidenced by their clustering near low values. The median bandwidth of I/V units at BL was 0.52 octaves (n ϭ 100), comparable with data from A1 of anesthetized cats (Schreiner et al, 2000;Moshitch et al, 2006) or awake macaques (Recanzone et al, 2000). In terms of Q values, I/V units exhibited a median Q 10 dB of 3.8 and median Q 40 dB of 1.8, similar to data from anesthetized cat A1 (Schreiner et al, 2000).…”

“…At BL, I/V and O units had median Q values of 2.7 and 5.9, respectively. The distributions of Q values at BL are shown in Figure 2 F. These data suggest that the selectivities of most neurons in A1 with respect to frequency and level tuning are much sharper than described previously (Calford et al, 1983;Recanzone et al, 2000;Schreiner et al, 2000;Moshitch et al, 2006).…”

“…However, these early studies did not systematically explore the relationship between frequency and level tuning, leaving open the possibility that nonmonotonic rate-level tuning is limited to frequencies at or near the best frequency (BF) of a unit. In the ϳ25 years since these studies, however, many studies have described FRAs of primary auditory cortex (A1) neurons as level intolerant in a variety of species such as cats (Calford et al, 1983;Heil et al, 1992;Schreiner et al, 2000;Moshitch et al, 2006), rats (Kilgard and Merzenich, 1999;Tan et al, 2004), mice (Linden et al, 2003), ferrets (Shamma et al, 1993), and macaque monkeys (Recanzone et al, 2000), to name a few. Except for the study by Recanzone et al (2000), the above cited studies were all conducted in anesthetized animals.…”