Dr. Abhishek Mandal, Ph.D.
Founder, Vision Science Academy, London, United Kingdom
Vision Science Academy Exclusive
What is Psychophysics?
Psychophysics is a broad, interdisciplinary field which correlates basic physical phenomena such as light, sound, taste, touch, and temperature with the sensory mechanisms implicated in the detection of these common sensory stimuli. This term was first coined by G.T. Fechner who elaborated the association between body and mind. With the advent of psychophysics, study of higher cognitive functions has also become a widely debated topic (Engen, 1988).
Applications in Vision Sciences
Being a multifaceted field of science, psychophysics also interacts with vision sciences. Psychophysicists tend to explore how the visual stimuli e.g., light, colour etc. are processed by the specialised neuronal centres inside our brain. In addition, colour contrast and brightness are other relevant elements of visual scenery which are potentially evaluated under the domain of psychophysics (Aleci & Rosa, 2022).
Visual Acuity & Psychophysics
One of the most commonly tested components of visual function is the estimation of visual acuity. This is mostly carried out by use of alphabetical charts (i.e., Snellen or ETDRS charts) (Kaiser, 2009). Psychophysics has established visual acuity as a tool to estimate the ability of the ocular system to differentiate between two distinct stimuli in the visual field. It is noteworthy that visual acuity seldom remains uniform in different retinal zones. The macula contains the fovea, a region which spans across 1-2 degrees and is characterised by maximum visual acuity (Bisley, 2011). The visual acuity falls significantly low while moving from the fovea to non-foveal regions and the latter generate a much cruder picture of the visual imagery. By implementing findings from visual acuity assessments, psychophysical analysts can broadly categorise various neuropsychological mechanisms which govern visual processing while also studying their intrinsic impairments. For example, a markedly reduced visual acuity serves as a classical sign of degenerative ocular pathologies, such as age-related macular degeneration (Christoforidis et al., 2011; Midena, 1989).
Visual Field & Psychophysics
Visual field assessment by means of perimetry also holds pivotal relationships with psychophysics. Perimetry allows the evaluation of various spatial patterns within the visual field (Phu et al., 2018). Kinetic perimetry is carried out by shifting a target from the peripheral vision into the central field at an approximate rate of 5 degrees per second. For targets which are located within the extent of 30 degrees from the centre, the shifting rate is slowed down to 2 degrees per second. Kinetic perimetry is effective in charting the three-dimensional mapping of blind spots or scotomas in a subject. As an alternative approach, the static version of perimetry has also been utilised by researchers which involves placing a motionless target at a particular spot within the visual field. By varying the intensity of light in the target’s background, the light threshold needed to accurately identify the target is determined. This helps pinpoint any areas in the visual field which are poorly sensitive to light (Johnson, 2013).
Studying the characteristic site, size, and pattern of such field defects plays a significant role in highlighting the underlying visual disorder. For instance, visual disorders e.g., chronic glaucoma, are typically associated with the development of tunnel vision. In this manner, principles of psychophysics can be implemented to allow for timely diagnosis and effective management of visual field defects.
References
Aleci, C., & Rosa, C. (2022). Psychophysics in the ophthalmological practice—I. visual acuity. Annals of Eye Science, 7, 37. https://aes.amegroups.com/article/view/6970
Bisley, J. W. (2011). The neural basis of visual attention. J Physiol, 589(Pt 1), 49-57. https://doi.org/10.1113/jphysiol.2010.192666
Christoforidis, J. B., Tecce, N., Dell’Omo, R., Mastropasqua, R., Verolino, M., & Costagliola, C. (2011). Age related macular degeneration and visual disability. Curr Drug Targets, 12(2), 221-233. https://doi.org/10.2174/138945011794182755
Engen, T. (1988). Psychophysics. In States of Brain and Mind (pp. 89-91). Birkhäuser Boston.
https://doi.org/10.1007/978-1-4899-6771-8_35
Johnson, C. A. (2013). Psychophysical factors that have been applied to clinical perimetry. Vision Research, 90, 25-31. https://doi.org/https://doi.org/10.1016/j.visres.2013.07.005
Kaiser, P. K. (2009). Prospective evaluation of visual acuity assessment: a comparison of snellen versus ETDRS charts in clinical practice (An AOS Thesis). Trans Am Ophthalmol Soc, 107, 311-324.
Midena, E. (1989). Psychophysics and visual aging. Metab Pediatr Syst Ophthalmol (1985), 12(1-3), 28-31.
Phu, J., Kalloniatis, M., Wang, H., & Khuu, S. K. (2018). Differences in Static and Kinetic Perimetry Results are Eliminated in Retinal Disease when Psychophysical Procedures are Equated. Transl Vis Sci Technol, 7(5), 22. https://doi.org/10.1167/tvst.7.5.22
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