Tools for physiology labs: an inexpensive high-performance amplifier and electrode for extracellular recording.

The cost of electronic equipment can be a critical barrier to including neurophysiology exercises in biology teaching programs. We describe the construction of a simple and inexpensive AC preamplifier with performance comparable to that of commercial products. The amplifier consists of two integrated circuits in five stages: differential input, fixed gain, variable gain (100 or 1000), low-pass filter (5 or 20 kHz), and 50 or 60 Hz notch filter. We compared our amplifier with two commercial units, the A-M Systems Model 1700 and the Grass P15. The quality of extracellular recording from a typical student preparation (spontaneously active crayfish motor nerve) was the same for all three amplifiers, although our amplifier has slightly higher internal noise than the P15 and slightly lower common-mode rejection than the 1700 and P15. In addition, we describe a simple suction electrode for extracellular nerve recording. It is easily constructed from readily available materials and uses a disposable plastic pipette tip, instead of the traditional glass tip, to contact the nerve. This tip is easily replaced if broken or clogged, and can be adapted to different recording conditions by selecting a different tip size or stretching the plastic. Development of this equipment is part of an ongoing project to promote neuroscience education by expanding the neurophysiology options available to laboratory instructors.

Spatial acuity of ultrasound hearing in flying crickets.

The minimum audible angle is the smallest angular separation at which two sounds are perceived as coming from distinct sources. To determine the spatial acuity of hearing in crickets, we measured minimum audible angles at various locations in azimuth and elevation. Crickets (Teleogryllus oceanicus) were able to discriminate between sources separated by 11.25 degrees (1/32 of a circle) in azimuth directly ahead of them; acuity declined to 45 degrees in azimuth for sound sources 90 degrees to the side and then improved to 33.75 degrees at the rear. Crickets were also able to discriminate between sources separated in elevation, although acuity was much poorer, ranging from 45 degrees at the front and rear of the animal to 90 degrees below the animal. A habituation-dishabituation test was used to test discrimination. This involved presenting a train of ultrasound pulses from one location, habituating the cricket's escape response. This train was followed by a test pulse of ultrasound from another location, after which a final pulse was presented from the original source. If the test pulse was discriminated from the habituating pulses, then the response to the final pulse was dishabituated. To determine the minimum audible angle, we repeated such tests while moving the two sound sources closer together until dishabituation no longer occurred.

Categorical perception of sound frequency by crickets.

Partitioning continuously varying stimuli into categories is a fundamental problem of perception. One solution to this problem, categorical perception, is known primarily from human speech, but also occurs in other modalities and in some mammals and birds. Categorical perception was tested in crickets by using two paradigms of human psychophysics, labeling and habituation-dishabituation. The results show that crickets divide sound frequency categorically between attractive (<16 kilohertz) and repulsive (>16 kilohertz) sounds. There is sharp discrimination between these categories but no discrimination between different frequencies of ultrasound. This demonstration of categorical perception in an invertebrate suggests that categorical perception may be a basic and widespread feature of sensory systems, from humans to invertebrates.

Demonstration of the precedence effect in an insect.

Field crickets are interesting models for study of auditory phenomena because they solve many of the same acoustic problems as humans, but with simpler nervous systems. Previous work in this lab and others has investigated sound localization, frequency and temporal pattern discrimination, habituation and dishabituation, and categorical perception. This paper demonstrates the precedence effect in crickets, using a standard two-pulse paradigm with a directional escape response to ultrasound. When two pulses of ultrasound are presented form opposite sides with a delay between, crickets respond only to the first pulse for delays of approximately 4 to 75 ms. For delays of 0 to 2 ms, the direction of response is variable (the first wave front does not have precedence); for delays over approximately 75 ms, crickets respond directionally to each of the two pulses. Some neural correlates of the precedence effect were studied by using this paradigm during recordings from a bilateral pair of ascending second-order auditory interneurons known to initiate ultrasound avoidance. There are no ipsilateral-contralateral differences in their responses that could account for the precedence effect; such interactions in the brain must be involved instead. This seems to be the first test of precedence effect in a nonmammal.