ALBERT

Notes: Summary 1. Central processing of electroreceptor, mechanoreceptor, and optic input in rays (primarilyTorpedo) and sharks (primarilyScyliorhinus) was studied by recording evoked potentials to both direct nerve shock and natural physiological stimulation. We found that electrosensory input has a widespread, complex central representation; convergence of different modalities occurs in the midbrain, and rays show some consistent differences from sharks in response dynamics. 2. Each modality shows distinct forms of evoked potential with a different dependence on recording locus and depth, and a different sequence of recovery, facilitation and depression on stimulus repetition. 3. InTorpedo, unlike sharks, the trigeminal nerve is quite distinctly divided into electrosensory (ampullary receptors) and mechanosensory (cutaneous) branches; inputs from these major branches have clearly separable central distributions and dynamics, with evoked responses to direct shock of the maxillary branch showing similarities to, and interactions with responses to d.c. fields in the water. 4. Electrosensory and mechanosensory responses in both rays and sharks demonstrate integrative properties already in the medulla, and optic responses also demonstrate early integration, with much longer latencies, in the retina. Prominent, complex, long-lasting responses occur in the tectum bilaterally, but greater contralaterally, with each modality having a different locus of maximum responses. Responses to each modality also occur less prominently, often with different latencies or dynamics, in the telencephalon, cerebellum, and structures deep to the tectum. 5. The various rays were consistently different from the sharks in having slower responses to each modality, much slower following capability to repetitive stimuli, and in being extremely resistant to electroshock convulsion. We suggest that the physiological differences in central sensory responses between these rays and sharks may be relevant in analyzing their different behavior.

Notes: Summary 1. Potamotrygon lacks ampullae of Lorenzini (defined by their long canals), otherwise general for elasmobranchs. There are present however microscopic ampullary organs with extremely short canals. A brief histologic description is provided, together with counts of their abundance in various parts of the body. They are chiefly concentrated ventrally in the head region. 2. The skin has a relatively high resistance compared to marine rays. This is measured in the physiologically significant way, by measuring the potential distribution in and around a living ray placed in a homogenous electric field. 3. The microscopical size of the ampullary organs and the high skin resistance are believed to be a specialization maintaining the electroreceptive function in the low conductivity, fresh water medium. 4. These rays are shown to be responsive to d.c. and low-frequency a.c. electric fields. They give specific movements seemingly related to feeding. They seem to be less sensitive than marine sharks and rays. The threshold stimulus is probably less than 120 μV/cm (corresponding to 0.03 μA/cm2 with water resistivity of 4 kOhm · cm). 5. Potamotrygon circularis appears to lack Savi's vesicles. However, an organ which may be equivalent is a tubular, subcutaneous, receptor in the Submandibular region. It does not open to the outside or connect to the skin or to the skeleton. Its spontaneous background nerve impulses and the increases in firing with mechanical stimuli are described.

Notes: Summary The physiology of mechanoreceptive lateral line areas was investigated in the thornback guitarfish,Platyrhinoidis triseriata, from medulla to telencephalon, using averaged evoked potentials (AEPs) and unit responses as windows to brain functions. Responses were analysed with respect to frequency sensitivity, intensity functions, influence of stimulus repetition rate, response latency, receptive field (RF) organization and multimodal interaction. 1. Following a quasi-natural vibrating sphere stimulus, neural responses were recorded in the medullary medial octavolateralis nucleus (MON), the dorsal (DMN) and anterior (AN) nucleus of the mesencephalic nuclear complex, the diencephalic lateral tuberal nucleus (LTN), and a telencephalic area which may correspond to the medial pallium (Figs. 2, 3, 13, 14, 15, 16). 2. Within the test range of 6.5–200 Hz all lateral line areas investigated responded to minute water vibrations. Best frequencies (in terms of displacement) were between 75 and 200 Hz with threshold values for AEPs as low as 0.005 μm peak-to-peak (p-p) water displacement calculated at the skin surface (Fig. 6). 3. AEP-responses to a vibrating sphere stimulus recorded in the MON are tonic or phasic-tonic, i.e., responses are strongest at stimulus onset but last for the whole stimulus duration in form of a frequency following response (Fig. 3). DMN and AN responses are phasic or phasic-tonic. Units recorded in the MON are phase coupled to the stimulus, those recorded in the DMN, AN or LTN are usually not (Figs. 5, 8, 9). Diencephalic LTN and telencephalic lateral line responses (AEPs) often are purely phasic. However, in the diencephalic LTN tonic and/or off-responses can be recorded (Fig. 11). 4. For the frequencies 25, 50, and 100 Hz, the dynamic intensity range of lateral line areas varies from 12.8 to at least 91.6 dB (AEP) respectively 8.9 and 92 dB (few unit and single unit recordings) (Fig. 7). 5. Mesencephalic, diencephalic, and telencephalic RFs, based on the evaluation of AEPs or multiunit activity (MUA), are usually contralateral (AN and LTN) or ipsi- and contralateral (telencephalon) and often complex (Figs. 10, 12, 16). 6. In many cases no obvious interactions between different modalities (vibrating sphere, electric field stimulus, and/or a light flash) were seen. However, some recording sites in the mesencephalic AN and the diencephalic LTN showed bimodal interactions in that an electric field stimulus decreased or increased the amplitude of a lateral line response and vice versa (Fig. 13B).

Notes: Summary Potentials were recorded from the epidermal head lines and from the CNS of young cuttlefish, Sepia officinalis, in response to weak water movements. 1. Within the test range 0.5–400 Hz a sinusoidal water movement elicits up to 4 components of response if the electrode is placed on a headline: (i) a positive phasic ON response; (ii) a tonic frequency-following microphonic response; (iii) a slow negative OFF response (Figs. 2, 5, 7A, 8, 11); and (iv) compound nerve impulses (Figs. 3A, 7B). 2. The amplitude of both the ON wave and the microphonic potential depends on stimulus frequency, stimulus amplitude and stimulus rise time (Figs. 4C, 6). Frequencies around 100 Hz and short rise times are most effective in eliciting strong potentials. The minimal threshold was 0.06 μm peak-to-peak water displacement at 100 Hz (18.8 μm/s as velocity). 3. Change of direction of tangential sphere movement (parallel vs. across the head lines) has only a small effect on the microphonic and the summed nerve potentials (Fig. 7). 4. Frequency and/or amplitude modulations of a carrier stimulus elicit responses at the onset and offset of the modulation and marked changes in the tonic microphonic response (Figs. 8, 9, 10, 11). 5. Evoked potentials can be recorded from the brain while stimulating the epidermal lines with weak water movements. The brain potentials differ in several aspects from the potentials of the head lines and show little or no onset or offset wave at the transitions of a frequency and amplitude modulation (Fig. 12).

Notes: Summary 1. Some fishes show a tendency to orient their ventral side towards a substrate and may thus tilt considerably when swimming near vertical walls or even under the ceiling of caves. This behavior was named theVentral Substrate Response (VSR) and was quantitatively studied in the black croaker (Cheilotrema saturnum, Sciaenidae) and in the weakly electric fishEigenmannia sp. (Rhamphichthyidae). 2. It was determined that theVSR ofC. saturnum is visually guided and that a vertical substrate can induce a tilt of about 64° away from the vertical if illumination is from above (Fig. 2). TheVSR ofEigenmannia sp. can be totally or predominantly guided by the electric sense of these animals and can induce ca. 30° tilt to a 45° tilted bottom (Fig. 4). 3. The amount of tilt displayed is dependent on the distance between the animal and the substrate. Measurable tilt responses inC. saturnum were observed up to a distance of 15 cm. 4. In a second experiment interactions between theDorsal Light Response (DLR) and theVSR were investigated inC. saturnum. It was found that tilt responses induced during aDLR or aVSR can add to each other when having the same direction or can subtract from one another, if opposite in direction. This experiment demonstrates the independence of theVSR- and theDLR-mechanisms. 5. After bilateral forebrain ablationC. saturnum did not show aVSR anymore. ADLR was still performed.

Notes: Summary 1. A preparation has been developed for the study of central auditory neurophysiology in cetaceans. Under N2O and Fluothane anesthesia, supplemented by succinylcholine chloride, twenty-nine specimens of four species of dolphins, most commonly Stenella caeruleo-alba, were stimulated with air-borne sound, water-borne sound, and directly by hydrophone pressed against the skin of the head. Evoked potential responses were recorded chiefly from the inferior colliculi but also from medullary auditory centers and the medial geniculate. 2. The recorded potentials were onset responses of complicated waveform, usually having components with latencies of 2–4 msec. The longer latency responses probably represent heterogenous input to the inferior colliculi and, in a few cases, activity of collicular cells or input to the medial geniculate body. Shorter latency responses probably represent output of the cochlear nuclei and activity of medullary auditory nuclei. 3. Although the evoked responses do not reflect increases in stimulus duration beyond a fraction of 1 msec, response threshold and waveform are sensitive to changes in rise time between 0.1 and 1.0 msec, and the small duration necessary to provide the onset response (as little as 0.1 msec) is also sufficient to provide accurate information about the frequency of the stimulus. 4. Evoked potential responses in certain locations and intensity ranges were found to change markedly in amplitude or waveform with intensity changes as small as 1 db. The same response in other intensity ranges might be little altered by intensity changes as large as 10 db. 5. Measurements of sensitivity as a function of frequency varied with electrode location, but averages or determinations of maximum sensitivity at each frequency at any location in a given animal produced audiograms in close agreement with that obtained behaviorally in Tursiops truncatus by Johnson (1966). Sensitivity was maximum at about 60 kc/s, and high between 20 and 70 kc/s; it typically fell about 25 db between 20 and 10 kc/s, and the same amount between 60 and 100 kc/s. The highest frequency eliciting a response was 120–140 kc/s. Data from the four species were not noticeably different. 6. Response waveform and amplitude were often changed much more dramatically than was threshold by small changes in stimulus frequency, emphasizing the heterogeneity of input to a given electrode location, and the sharpness of frequency discrimination. Changes in threshold of up to 3 db/percent change in frequency were seen. Actual frequency discrimination is undoubtedly much sharper than these evoked potential measurements would indicate. 7. The masking effect of background tones was shown to be confined to a relatively narrow band of frequencies surrounding that of the stimulus. Changes in masking effectiveness reached 30 db/10 kc/s change in background tone frequency. This too provides a conservative estimate of response “tuning”, since the evoked potentials represent the summed activity of such a large population of different units. 8. Frequency modulated (FM) pulses of 2–5 msec duration and gradual rise and fall times were found sometimes to produce unexpectedly large responses compared with the response to any constant frequency (CF) pulse within the swept range. The response waveforms differed sharply with changes in starting frequency or range swept, even when the maximum response amplitude remained unchanged. In certain cases an FM pulse was found to be up to 10 db more effective if swept upward than if swept downward, and vice versa. It seems probable that significant populations of cells exist that are specifically responsive to sounds having certain FM characteristics, or certain combinations of frequencies in a given temporal order. 9. Temporal resolution of successive sounds by cetaceans was found to be extremely rapid; virtually complete recovery of responsiveness in the fastest case took 0.8–1.0 msec between clicks; some initial recovery was often visible at approximately 0.5 msec. Following of a train of click stimuli was seen at repetition rates up to 2,000/sec. However, more typical recordings showed complete recovery of the collicular evoked potential required 3–5 msec. There was no evidence of facilitation of response to the second of two identical stimuli, as has been observed in echolocating bats. 10. Distortion of the sound field by holding a sheet of paper between the source and the lower jaw or melon can in some recording sites cause dramatic changes in response waveform without significantly affecting the maximum response amplitude. 11. The primary pathway of sound to the cochlea is via the lower jaw. When stimuli were applied either with a loudspeaker in air, a hydrophone pressed against various points on the head, or a hydrophone underwater, sensitivity was consistently greatest over a limited portion of the side of the contralateral lower jaw, nearly as high over the ipsilateral side of the melon, markedly lower over the side of the head including the external ear orifice, and almost nil at the rostrum. This map was confirmed by using a distant sound source and local acoustical shields of 10×12 cm pieces of foam rubber or paper; held over the sensitive part of the lower jaw or melon this caused marked attenuation of response. At low frequencies (30 kc/s) the same pattern was found, but sensitivity was somewhat more uniform, relatively greater over the side of the head including the external auditory meatus and reduced on the melon, compared with the high-frequency pattern (60 kc/s). 12. Gentle stroking of the side of the jaw or splashing of water on the jaw masked response to sounds. The effective input is acoustic. The effective area is circumscribed and identical to that for sound. Electrical stimulation of the skin of the jaw elicited no response in auditory centers and did not mask responses to sounds. 13. A cone of best reception is directed forward and downward, some 5-30° from the midline in the horizontal plane (the higher the frequency the closer to the midline), and 5–20° below the horizontal in the optimum vertical plane. Sensitivity fell sharply (up to 1.5 db/degree) at more lateral angles. Sound coming from above the horizontal is probably much reduced in effectiveness. 14. None of the response parameters studied was found to be affected by the anesthetics used, or by anesthetic doses of Nembutal. 15. The evoked response characteristics are evaluated from the standpoint of adaptations for echolocation, and compared with analagous responses in bats and non-echolocating mammals. Possible mechanisms of distance measurement are considered. Target localization is discussed in light of the patterns of angular sensitivity resulting from use of the lower jaws as primary pathways of sound to the ears. The question is raised: In view of the extreme sensitivity of the head surface to mechanical disturbance, especially around the lower jaw, is echolocation usable during natural activity, e.g. rapid swimming, pursuit of prey, or food mastication ? If so, what adaptations have occurred to help overcome the potential masking noise ?