# Threshold time for stimulus to be consciously perceived. Why?

A visual stimulus must last longer than some threshold duration to be perceived consciously. For example, a light dot flashed for a duration of 10ms can't be consciously perceived. Why? What prevents a sub-threshold flash to be consciously perceived? Does this mean the relevant neurocircuits have to keep on firing for some threshold time in order to reach consciousness? If so, why?

• because it takes a certain amount of processing time to be sure of what you've seen – honi Nov 18 '15 at 15:37

If by consciously perceived, you mean recognizing a feature (and possible reactions accordingly), there are several tasks to be considered that seem to cause delays:

1. Saccades and eye movements: During saccades we are blind, so we must wait a little to bypass a saccade so participants are able to see. It is called saccade masking.

2. Transmission: neurons are not that fast (around 100m/s for myelinated ones), so transmitting action potential from retina to the primary visual cortex takes time, of around hundreds of milliseconds.

3. From visual cortex to other area of the brain (for verbal responses, reasoning, matching, or further recognitions that are not simply visual features),

4. Transmitting reaction signals from motor cortex to the hand so participants can respond.

These are just neurological pathways. Considering some other cognitive shortcuts (e.g. schemas and categories), participants need to bypass their default reasoning modes (unconscious?) that are learned by training, stereotypes, or being stimulated so long that it's been moved from foreground to the context (speaking of perception without awareness).

• I think you misunderstood me. For instance, a light dot flashed for a duration of 10ms can't be perceived consciously, even if you waited for all the time it takes for the signal to travel over the brain. People always report they see nothing. Why? – Eric Nov 18 '15 at 16:22
• as i said above, because it takes time to perform the processing that determines what the stimulus is – honi Nov 18 '15 at 16:27
• @honi, but you can wait for all the time it takes to perform processing after the stimulus is flashed, there's still no consciousness of it. Why? – Eric Nov 18 '15 at 16:32
• because at least some of the processing requires presence of the stimulus. – honi Nov 18 '15 at 17:42
• in other words it is less "what prevents a sub-threshold duration flash from being perceived" and more "what is necessary for the mechanisms for bringing visual stimuli into perception?" There will obviously be some threshold, the question is simply "what is the threshold?" – honi Nov 18 '15 at 17:50

Depending on how you define stimulus duration, the auditory system likely has no minimum stimulus duration needed for conscious perception. We are quite sensitive to "clicks". A "standard" click duration of 20 microseconds (a single sample at 50 kHz) is not uncommon. Decreasing the duration below this would simply require better stimulus generation hardware (faster sample rate and higher bandwidth transducers). Decreasing the duration would simply add high frequency energy that falls outside the audible range.

An ideal impulse with infinite bandwidth will have a duration of zero (and a magnitude that is a pain to describe), but a real transducer with finite bandwidth will result in some finite duration. Further the stimulus will be mechanically filtered by the outer, middle, and inner ear further decreasing the bandwidth (aka increasing the duration). Finally in the inner ear the signal will be transformed from a mechanical/acoustic signal to an electrical signal with a duration that depends on the place in the brain that you measure the electrical signal.

• Interesting. Let's put the duration of perceivable visual stimulus into perspective. Ultraviolet light on the verge of visibility has wavelength of 400nm, or a frequency of about $8*10^{14}$Hz. This means any stimulus with duration longer than about $10^{-14}$ second can be made to lie within the visibility range, although you need a duration of $10^{-5}$ second in order to perceive it! An order of $10^9$ times longer! – Eric Feb 21 '17 at 6:55
• But 50kHz is way beyond audible frequency, can we really hear a click in that range? en.wikipedia.org/wiki/Audio_frequency – Eric Feb 21 '17 at 7:12
• @Eric you are misunderstanding what frequencies are in an impulse. A true impulse has all frequencies and zero duration. As you increase the impulse duration from 0 to 20 microseconds, you lose the frequencies above 25 kHz, which as you say are not audible. – StrongBad Feb 21 '17 at 13:01