What is the absolute threshold for perception research?

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Abstract

At the absolute threshold of vision, Hecht, Shlaer and Pirenne estimate that 5–14 photons are absorbed within a retinal area containing ~500 rods. Other estimates of scotopic threshold vision based on stimuli with different durations and focal areas range up to ~100,000 photons. Given that rod density varies with retinal eccentricity and the magnitude of the intrinsic noise increases with increasing stimulus area and duration, here we determine whether the scotopic threshold estimates with focal stimuli can be extended to full-field stimulation and whether summation explains inter-study differences. We show that full-field threshold vision (~1018 mm2, 10 ms duration) is more sensitive than at absolute threshold, requiring the absorption of ~1000 photons across ~91.96 million rods. A summation model is presented integrating our and published data and using a nominal exposure duration, criterion frequency of seeing, rod density, and retinal area that largely explains the inter-study differences and allows estimation of rods per photon ratio for any stimulus size and duration. The highest signal to noise ratio is defined by a peak rod convergence estimated at 53:4:1:2 (rods:rod bipolar cells:AII amacrine cells:retinal ganglion cells), in line with macaque anatomical estimates that show AII amacrine cells form the bottleneck in the rod pathway to set the scotopic visual limit. Our model estimations that the rods per photon ratio under full-field stimulation is ~3000X higher than at absolute threshold are in accordance with visual summation effects and provide an alternative approach for understanding the limits of scotopic vision.

A quantitative approach to an understanding of fluctuating phenomena encountered near the threshold of vision has involved an analysis of frequency of seeing curves. The results of various investigators are discussed with reference to the possibility of an explanation based exclusively on the statistical fluctuations in the stimulating energy. Psychological variability in the conditioning of the subject can account for controversial results. The integrative actions in the visual system measured with the aid of flashes containing two separate small and short stimuli—with variable time and distance apart from each other—were discussed. Various merits of such experiments are put forward. The absolute threshold behavior with respect to dependences of threshold energy on duration, velocity of moving sources, length and width of line-shaped targets, diameter and size of circular and square targets, was confronted and found in agreement with the two quanta explanation. Objections and modifying suggestions of other investigators on the two quanta concept are discussed. It is concluded that experimental results deny the correctness of such objections or modifications. The brightness impression of the perceived flash under the various conditions decreases with increasing target-size or test area. Suggestions are made as to the mechanisms which accounts for this effect. Such mechanisms account for deviations from theory at the simultaneous occurrence of large and long test flashes.

© 1955 Optical Society of America

The process of sensation involves transmission of information to the brain by the sensory receptors like nose, eyes, ears, mouth and skin. Sensation and perception though are related, but are considered to be as distinct processes. In case of sensation, sensory receptors play a key role in transmitting the inputs to the brains by converting these inputs to neural impulses. Perception is the process by way of which the brain in the light of past experiences, memory and learning selects and interprets sensations. Perception of the same senses will vary from person to person because of the differences in individual’s memory, learning, expectations and emotions.

The Senses

Humans have five key senses like sight, smell, taste, hearing and touch. Apart from these five senses, these day scientists have equally acknowledged the importance of the other two senses, vestibular senses and kinaesthesia. Kinaesthesia is about body awareness, whereas vestibular senses help us detecting gravity, linear or rotary acceleration (such as speeding up or slowing down in a straight or a curved road). In a way, both Kinaesthesia and Vestibular senses help us in balancing.

In the process of sensation, sensory receptors help in the conversion of the energy into neural impulses. The neural impulses on reaching the cerebral cortex of the brain, act as small microprocessors as a result of which sensation and perceptual process takes place.

The Sensory Absolute Threshold

Threshold establishes or sets forth the minimum level for any event to take place. In psychophysics or neuroscience, the sensory threshold can be of various types.

1. The Recognition Threshold: It is the minimum level or limit of threshold within which a stimulus may not only be detected but can also be recognized as well.
2. The Differential Threshold: It is that level of threshold in which the difference which is detected in a stimulus can be easily recognized or perceived.
3. The Terminal Threshold: The threshold limit beyond which a stimulus may no longer be recognized or perceivable.
4. The Sensory Absolute Threshold: It is the smallest detectable limit of a sensory stimulus. It can also be described as the lowest intensity at which a stimulus can be recognized or detected. An example for explaining the absolute threshold could be through odour test, in which a fragrance is released into the atmosphere. The absolute threshold in this case would be the least possible fragrance which will be enough for a subject to recognize or detect the odour.

Factors which Influence the Absolute Threshold

Various factors influence sensory absolute threshold stimulus sensory adaptation and individual expectations or motivation (cognitive processes).

Sensory Adaptation: Sensory adaptation takes place when the sensory receptors are no longer being able to perceive the stimulus because of the continuous contact of our senses with the stimulus. For example, if you dip your finger in a bowl filled with warm water, after some while you will no longer will be able to feel that much warmth as your senses have adapted with the stimulus. Sensory adaptation can take place with any of our senses just like thresholds. It is also called as neural adaptation and can be explained as a gradual reduction or a fall in the sensory response because of the repeated exposure of a particular stimulus over a period of time. This change can be both positive and negative and does not necessarily involve ignoring of the stimulus completely. One more example for explaining the sensory adaptation process could be, we no longer feel the clothes once we are dressed or forget the sounds of TV or radio while focusing on some pressing or important work.

Sensitization: On the other hand, the process of sensitization can be described as a gradual increase in the behavioural response over a period of time due to the regular exposure of a given stimulus. Contrary to sensory adaptation process, in which the exposure of a given stimulus is required in large volume for deriving a proper response or a change in the behaviour to take place, in case of sensitization, the stimulus is required in small quantity for eliciting a response or a significant behavioural change.

Cognitive Processes: A person’s motivation or expectation equally plays a deciding role whether a stimulus can be recognized or perceived at a minimum threshold level.

Sensory adaptation and sensitization are an integral aspect of the life and personality. We come across many forms of stimuli on a day to day basis in our life, some of which is recognized and perceived, while some we gradually forget or ignore which may include some specific sounds, smell or image exposures.

Sensory Difference Thresholds

Just Noticeable Difference is the minimum possible change in the sensory stimulus required by a person to recognize that the change has taken place. It is the smallest recognizable difference between the start and the secondary level of the stimulus. It is also known as the differential threshold or difference limen. Differential threshold was first explained by experimental psychologist and physiologist Ernst Weber and the concept was further expanded by Gustav Fechner. JND applies to a variety of senses like hearing, smell, sight, taste and touching. It can also be applied to diverse situations or things like noisiness, brightness, weight, sweetness, pressure, etc.

It is equally important not to treat Just Noticeable Difference and Absolute Threshold as similar concepts. Differential Threshold explains the perceivable difference in the stimulus exposure, while Absolute Threshold focuses on the smallest detectable stimulation. The absolute threshold in case of sound will be the lowest sound volume that a person will be able to detect. On the other hand, Just Noticeable difference will be the change in the volume of sound which a person will be able to detect or recognize.

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