A new psychophysical law has been discovered by researchers in Lisbon, Portugal, based on the time it takes subjects to perceive different stimuli.
By simultaneously playing two distinct sounds to subjects - one slightly louder than the other - the researchers observed new findings that, they say, provide a mechanistic explanation for the 200-year-old Weber's Law.
Psychophysics was started by a seemingly simple observation: Professor Ernst Heinrich Weber asked subjects to say which of two slightly different weights was heavier. He then gave the subjects two heavier sets of weights with the same slight weight difference.
In his experiments, Weber discovered that the probability that a subject will make the right choice depends solely on the ratio between the weights.
As an example: a subject who is correct 75% of the time, when comparing a weight of 1 Kg and a weight of 1.1 Kg, will be correct 75% of the time when comparing two weights of 2 and 2.2 Kg. The ratio of 10% governs their perception.
His findings showed that perceived change in stimuli is proportional to the initial stimuli. This was true across different senses, such as vision, hearing, touch, and smell.
Take this visual representation of Weber's Law as an example:
On each side, the bottom square contains 10 more dots than the upper one. Despite this, the perception is different: the difference for the 10 and 20 squares is easily perceivable, while the two squares on the right look the same.
Weber's simple, yet precise, observation showed that our brains follow precise, quantifiable behaviors of perception that can be understood via mathematical laws.
Essentially, Weber's Law started the field of Psychophysics by showing that we could study brain processes and perception with mathematical models - much in the same way we use models to understand the laws of physics governing the universe.
A new law based on time
Now, a team of researchers at the Champalimaud Centre for the Unknown in Lisbon, Portugal, has discovered a new law that explains why perception is tied to initial stimuli, as per Weber's Law.
The team showed that the time that it takes to make a choice, not only the outcome of the decision, governs Weber's Law. Their findings are described in a paper in the scientific journal Nature Neuroscience.
In order to reach this conclusion, the study's principal investigator, Alfonso Renart, and his team, trained rats to discriminate between two sounds of slightly different variation in decibels.
They constructed miniature headphones for the rats - yes, you read that correctly - and used these to deliver the two sounds simultaneously to the rats' two separate ears.
Rats naturally orient their head towards a louder sound. This meant that the researchers could see which sound the animals perceived as the loudest thanks to their reaction. Crucially, the researchers recorded the time it took to make the decision, as well as the decision itself.
Though the rats' decisions confirmed that the animals' behavior matched Weber's Law, it was the analysis of the time readings that was critical.
Pardo-Vazquez, one of the study's co-authors, explains in a press release:
"Typically, studies of Weber's law focussed on the accuracy of the discrimination, which is what Weber himself described. Surprisingly, the time taken to decide has received little attention."
The team realized that the decision times and the loudness of the sounds were linked: the louder the two sounds, the shorter the decision time. Decision times were shown to be exactly proportional, regardless of the sound intensity, as long as the difference in intensity was constant.
They followed up this research with experiments on human subjects and further experiments on rats, using odor perceptions - all of these backed up the initial findings.
The team discovered a new "psychophysical law", which they refer to as 'Time-Intensity Equivalence in Discrimination' (TIED). TIED, the researchers say, holds the key to Weber's Law, as it can be used to calculate a mathematical model to explain Weber's Law.