Clementine Thurgood

Towards a visual recognition threshold: New instrument shows humans identify animals with only 1 ms of visual exposure

The human visual system is very adept at extracting categorical information from complex scenes with only the briefest of exposure. Here we show that information from visual scenes can be processed to the level of identification with formally unattainable, ultra-brief (1ms) presentations. This brief presentation time is afforded by a new instrument, the light-emitting diode (LED) tachistoscope, in which a liquid crystal display (LCD) is illuminated externally by a brief LED flash after LCD steady-state is reached, such that image onset and offset timing can be precisely controlled. Photographs of animals were presented with or without backgrounds for 1ms and 10ms. The results indicate that visual recognition of objects benefits from presenting them in isolation rather than with a background at smaller (1ms) durations. In both conditions, however, animals could be recognised at 1ms at least 83% of the time, possibly due to iconic memory and top-down, feedback mechanisms.

Development of a light-emitting diode tachistoscope

This paper describes a new method for rapid visual stimulus delivery, the light-emitting diode (LED) tachistoscope. An array of white LEDs provided a luminous intensity greater than 1,000,000 mcd. This array was placed behind a liquid crystal display (LCD) to function as a backlight; switching it on and off determined the visibility of the display. Commands to illuminate for periods from 1 ms to continuously on were relayed from a computer to the LED array. Changes in luminous intensity at the surface of a LED and the LCD were recorded via oscilloscope. The required duration of light pulses consistently matched the durations displayed, with only microsecond discrepancies due to turn-on and turn-off delays. Images were illuminated on the LCD screen for as little as 1 ms, with the amplitude of the luminance consistent across trials. The LED tachistoscope can be used with any computer to display images extremely briefly, potentially at the submillisecond level, providing superior performance to traditional and computer monitor tachistoscopes.