In an earlier study, [Sandel, Teas, Fedderson, and Jeffress, J. Acoust. Soc. Am. 27, 842 (1955)] the writers attempted to correlate interaural intensity and time differences with the subject's localization response. The present paper is an extension of that work. It includes physical measurements of interaural time differences and intensity differences, and attempts to relate these differences to the localization response at a variety of frequencies. The stimuli to be localized were provided through earphones, and the subject was required to match the position (in his head) of a noise and a tone. The noise to one ear was delayed, and the tone presented with no time or phase difference. The subject adjusted the interaural level of the tone until it and the noise appeared to be in the same place. The two were presented alternately by means of a gate having a 150-msec rise and decay time. Data from the localization judgments were then compared with the findings from the acoustical measurements of time and intensity. Several systematic trends were found and compared with results of the previous study.

Three experiments on the localization of air-borne sound are described. They were conducted in an anechoic room and employed an acoustical "pointer" as the subject's method of indicating the direction of the stimulus tone. The pointer was a small loudspeaker carried on a boom which rotated about a vertical axis through the subject's head. This speaker presented a wide-band noise which alternated with the tone to be localized. The switching was transientless and was performed by an electronic gate having a 100-millisecond rise and decay time.Three small loudspeakers in enclosures, one mounted directly in front of the subject and one 40 ø to each side, presented the stimulus tones to be localized. In the first experiment the speakers were employed singly. In the other two experiments they were used in pairs. In the second experiment the pairs of speakers were in phase; in the third, they were in phase opposition.The stimulus conditions of Experiment 2 generate a "phantom source" which appears to lie between the two speakers employed. The predicted location is compared with the subjects' responses.The stimulus conditions of Experiment 3 generate a phantom source which according to prediction, should lie toward the side opposite to the asymmetrically placed speaker and should move in direction with frequency. This prediction is borne out by the subjects' responses for frequencies where interaural time is the dominant basis for localization.The results of the three experiments support the conclusion that the localization of tones below about 1500 cps is determined largely by interaural time differences. For frequencies above 1500 cps differences of intensity at the two ears must be the dominant factor, but neither probe microphone measurements nor audiograms for the subjects provided information from which satisfactory predictions could be made.

Many of the phenomena of masking can be explained on the basis of two models, one for monaural listening, and the other for binaural. The monaural model is the familiar narrow band-pass filter followed by a detector responsive to changes in output level. The binaural model is a series of coincidence detectors associated with a delay-network capable of matching delay in the stimulus with a delay in the neural path. The two models have proved helpful in understanding the phenomena of tonal masking, and have led to a number of predictions which have been subsequently verified by experiments reported here. Some of the new findings are related to monaural masking and some to binaural. Among the latter are the fact that masking-level differences can be observed in the masking of one pure tone by another when a short signal is employed, and that a binaural signal can be heard in the presence of uncorrelated noise at the two ears better than a monaural signal can be heard against noise presented monaurally, again provided that a short signal is employed. Many studies of the masking of tones have appeared during the past few years. They have been characterized by close agreement in results wherever they have had common conditions, and they provide a large body of dependable quantitative information. With this body of fact there also grew, at least in the minds of present writers, a considerable state of bewilderment. Many of the facts appeared to contradict inferences drawn from others. The present paper discusses a number of these facts and attempts to encompass them under a few explanatory principles.

Abstract. Unit electrical activity was recorded from single neurons in the lumbo-sacral spinal cord of 15-, 17-, and l9-day chick embryos, in situ. The dorsal columns showed relatively continuous single-unit activity. Below this lies an area of relative quiet 100-200,t deep. The ventral two thirds of the cord was the most active region, being characterized by polyneuronal bursts and intermittently active single units.The origin of motility of the chick embryo has been the subject of considerable conjecture. Kuo,l Schneirla,2 and others have proposed that behavior of the embryo is caused by stimulation. Hamburger,3 in contrast, has hypothesized that embryonic motility results from endogenous activity within the spinal cord. The available evidence supports the hypothesis of spontaneous (nonreflexogenic) motility. Motility, from its onset at 31/2-4 days up to 7 days of incubation, is nonreflexogenic because no adequate external stimulus is effective in evoking a response.4 The nonreflexogenic nature of motility in embryos older than 7 days has been demonstrated by experiments utilizing spinal embryos which were deafferented by the removal of the dorsal half of the spinal cord at two days. These embryos showed normal patterns of leg motility at least up to 15 to 17 days.5 Although the brain influences normal embryonic behavior,6 and sensory input may influence it, these recent experiments imply that autonomous activity of interneurons or motor neurons in the ventral half of the cord is sufficient to sustain motility up to about 15 to 17 days. On the 17th day, the spontaneous, uncoordinated motility of early stages declines and is superseded by a series of coordinated movements which lead to hatching of the chick on day 21.7 Sensory and brain inputs may be necessary for the execution of these late appearing, well organized movements.8The behavioral experiments discussed provide only indirect evidence concerning the neural substrate of embryonic behavior. The neurophysiological basis of motility can be revealed by direct examination of the activity of the spinal cord of the embryo. Given an adequate description of the patterns and distributions of single-unit activity in the normal embryonic spinal cord, it should be possible through experimental intervention to show the relative contributions to behavior of sensory, brain, and local cord inputs. The purpose of this preliminary paper is to provide the required description of the normal activity of single neurons in 508

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