Dynamic Visual Acuity

We present a review on the visual ability to discriminate fine details of moving objects (DVA: Dynamic Visual Acuity), showing the most relevant differences, which have been attributed to this visual capacity in comparison to SVA (static visual acuity). It is known that the correlation between SVA and DVA is low. Moreover, when DVA is measured, not only the minimum spatial separation that the visual system can resolve is evaluated, but also the functionality of the oculomotor system. Therefore, assessing DVA also involves measuring the ability of the eye to actively seek information. Nowadays, it is known that DVA is one of the best indicators of success in certain sports specialties (table tennis, baseball, etc...) and that it negatively correlates with accident rates in traffic scenarios. The investigated factors that produce a significant reduction in dynamic spatial resolution are: (a) the speed of the stimulus, affecting both vertical and horizontal trajectories; (b) the stimulus exposure time; (c) ambient illumination; (d) reduction in contrast and (e) subject age. Moreover, it has been verified that this visual capacity is likely to improve with training.

Keywords:
Static visual acuity; dynamic visual acuity; visual abilities; movement perception; visual psychophysics

Authorship

Lluïsa Quevedo

Universitat Politècnica de Catalunya, Barcelona, EspanhaUniversitat Politècnica de Catalunya, Barcelona, EspanhaUniversitat Politècnica de Catalunya, Barcelona, Espanha

http://orcid.org/0000-0002-0976-9372

J. Antonio Aznar-Casanova ^** Mailing address: Universitat de Barcelona, Facultad de Psicología, Passeig de la Vall d’Hebron, 171, Barcelona, 08035, Spain. Phone: +34 933 125 145; Fax: +34 934 021 363. E-mail: jaznar2@ub.edu

Universitat de Barcelona, Barcelona, EspanhaUniversitat de Barcelona, Barcelona, EspanhaUniversitat de Barcelona, Barcelona, Espanha

http://orcid.org/0000-0002-3084-3135

José Aparecido da Silva

Universidade de São Paulo, Ribeirão Preto, SP, BrasilUniversidade de São Paulo, Ribeirão Preto, SP, BrasilUniversidade de São Paulo, Ribeirão Preto, SP, Brasil

http://orcid.org/0000-0002-1852-369X

* Mailing address: Universitat de Barcelona, Facultad de Psicología, Passeig de la Vall d’Hebron, 171, Barcelona, 08035, Spain. Phone: +34 933 125 145; Fax: +34 934 021 363. E-mail: jaznar2@ub.edu

SCIMAGO INSTITUTIONS RANKINGS

Universitat Politècnica de Catalunya, Barcelona, EspanhaUniversitat Politècnica de Catalunya, Barcelona, EspanhaUniversitat Politècnica de Catalunya, Barcelona, Espanha

Universitat de Barcelona, Barcelona, EspanhaUniversitat de Barcelona, Barcelona, EspanhaUniversitat de Barcelona, Barcelona, Espanha

Universidade de São Paulo, Ribeirão Preto, SP, BrasilUniversidade de São Paulo, Ribeirão Preto, SP, BrasilUniversidade de São Paulo, Ribeirão Preto, SP, Brasil

Figures

Figures (7)

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Figure 1
An optical card (Optotype) similar to a Snellen chart that observers should look at from a certain distance. The method to determine static visual acuity consists of presenting the observer with dots, lines or rings with a certain separation between them, which decreases until the visual acuity limit (psychophysical method of limits GT Fechner) is achieved. The limit of spatial resolution (the smallest size) that the subject can visually resolve is reached when he is unable to identify the letters of a row (or perceive the distance between two points or lines or the opening of a ring). The absolute threshold for spatial resolution or visual acuity (VA) is usually found close to 0.5 arc-minutes of visual angle. To have a reference, for example, a letter of 1 cm in height subtends a visual angle of 1 degree when viewed from a distance of 57 cm. The (Vernier) Acuity for humans is approximately 5-10 arc-seconds of visual angle.

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Figure 2
Calculation of the Static Visual Acuity (SVA) in a Snellen-type optotype. It is assumed that the observer looks at the letter from a 5 m distance (d = 5m) and that, therefore, the height of the letter will be 7.25 mm and the thickness of the horizontal feature will be s = 1.45 mm (s = size).

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Figure 3
Three optotypes used in measuring visual acuity (VA): (1) the classic Landolt’s ring, (2) the Palomar universal ring disk, and (3) a Gabor patch. The Landolt’s ring is a circular optotype with a discontinuity or gap. To assess AV with this eye chart, the observer is asked to look at each row of the optical card and has to indicate in which orientation is the discontinuity. The Palomar universal ring-disc (Palomar, 1991Palomar, F. J. (1991). Anillo-Disco Palomar: Optotipo Universal para determinar la agudeza visual. Ver y Oír, 61, 29-35.) has the advantage that it subtends the same angle in all retinal meridians. Note that the peripheral white disc and black central circle correspond in every size to the minimum separable and among all its strokes there is always the same separation. This is the reason why with this optotype visual acuity can be calculated with high precision. The figure shows the ring-disc inscribed in a square divided into five horizontal and five vertical parts where we can see that all its elements subtend the same angle. The right panel of the figure shows an illustration of a Gabor patch which could be used as optotype-stimulus with an orientation of 135 degrees. The luminance of the background is equal to the mean luminance of the Gabor function. This is a grey level representation of the 2-D Gabor function. Of course, the Gabor patches could vary in both spatial frequency and speed or temporal frequency with patch drifting. Participants must indicate the orientation of the stripes.

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Figure 4
Function that represents the highest spatial frequency that a human subject can discriminate in a certain range of speeds for different contrasts (Taken from Aznar-Casanova et al., 2005Aznar-Casanova, J. A., Quevedo, L. 1., & Sinnet, S. (2005). The effects of drift and displacement motion on dynamic visual acuity. Psicologica, 26,101-126.).

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Figure 5
Bernell’s Rotator is frequently used for the clinical evaluation of DVA, especially in a Sport Optometry context.

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Figure 6
The DynVA is a computer software designed to assess DVA. The researcher can select the optotype to be presented in the two forms of the test: (a) Size Series; (b) Speed Series. In the Size series, the optotype size remains constant, while its speed decreases as it moves across the screen. Conversely, in the Speed series, the optotype speed remains constant while its size increases progressively as it moves across the screen. In both series, the trial ends when the subject presses the arrow key that matches the position of the band gap of the test card.

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Figure 7
How does compute the speed of the stimulus on the retina? To compute DVA, the software first calculates the speed of the stimulus on the screen according to the following formula: Speed (in m/s) = Frame rate (in Hz; frames/s) * Dot_pixel (in m) * Step (in pixels). Then, to calculate at what speed the stimulus moves across the retina (to obtain the speed in degrees of visual angle), two factors must be taken into account: (1) the speed of the target on the screen, in meters per second, and (2) the viewing distance (usually in m): Speed on the retina = arc tan (on screen_speed / Viewing distance). For example, given a target speed on the screen of 0.420 m/s and a viewing distance of 2 m, the speed would be 11.86°/s of visual angle. The DVA is then expressed as the smallest detail that the observer is able to perceive at that speed.

Figure 1 An optical card (Optotype) similar to a Snellen chart that observers should look at from a certain distance. The method to determine static visual acuity consists of presenting the observer with dots, lines or rings with a certain separation between them, which decreases until the visual acuity limit (psychophysical method of limits GT Fechner) is achieved. The limit of spatial resolution (the smallest size) that the subject can visually resolve is reached when he is unable to identify the letters of a row (or perceive the distance between two points or lines or the opening of a ring). The absolute threshold for spatial resolution or visual acuity (VA) is usually found close to 0.5 arc-minutes of visual angle. To have a reference, for example, a letter of 1 cm in height subtends a visual angle of 1 degree when viewed from a distance of 57 cm. The (Vernier) Acuity for humans is approximately 5-10 arc-seconds of visual angle.

Figure 2 Calculation of the Static Visual Acuity (SVA) in a Snellen-type optotype. It is assumed that the observer looks at the letter from a 5 m distance (d = 5m) and that, therefore, the height of the letter will be 7.25 mm and the thickness of the horizontal feature will be s = 1.45 mm (s = size).

Figure 3 Three optotypes used in measuring visual acuity (VA): (1) the classic Landolt’s ring, (2) the Palomar universal ring disk, and (3) a Gabor patch. The Landolt’s ring is a circular optotype with a discontinuity or gap. To assess AV with this eye chart, the observer is asked to look at each row of the optical card and has to indicate in which orientation is the discontinuity. The Palomar universal ring-disc (Palomar, 1991Palomar, F. J. (1991). Anillo-Disco Palomar: Optotipo Universal para determinar la agudeza visual. Ver y Oír, 61, 29-35.) has the advantage that it subtends the same angle in all retinal meridians. Note that the peripheral white disc and black central circle correspond in every size to the minimum separable and among all its strokes there is always the same separation. This is the reason why with this optotype visual acuity can be calculated with high precision. The figure shows the ring-disc inscribed in a square divided into five horizontal and five vertical parts where we can see that all its elements subtend the same angle. The right panel of the figure shows an illustration of a Gabor patch which could be used as optotype-stimulus with an orientation of 135 degrees. The luminance of the background is equal to the mean luminance of the Gabor function. This is a grey level representation of the 2-D Gabor function. Of course, the Gabor patches could vary in both spatial frequency and speed or temporal frequency with patch drifting. Participants must indicate the orientation of the stripes.

Figure 4 Function that represents the highest spatial frequency that a human subject can discriminate in a certain range of speeds for different contrasts (Taken from Aznar-Casanova et al., 2005Aznar-Casanova, J. A., Quevedo, L. 1., & Sinnet, S. (2005). The effects of drift and displacement motion on dynamic visual acuity. Psicologica, 26,101-126.).

Figure 5 Bernell’s Rotator is frequently used for the clinical evaluation of DVA, especially in a Sport Optometry context.

Figure 6 The DynVA is a computer software designed to assess DVA. The researcher can select the optotype to be presented in the two forms of the test: (a) Size Series; (b) Speed Series. In the Size series, the optotype size remains constant, while its speed decreases as it moves across the screen. Conversely, in the Speed series, the optotype speed remains constant while its size increases progressively as it moves across the screen. In both series, the trial ends when the subject presses the arrow key that matches the position of the band gap of the test card.

Figure 7 How does compute the speed of the stimulus on the retina? To compute DVA, the software first calculates the speed of the stimulus on the screen according to the following formula: Speed (in m/s) = Frame rate (in Hz; frames/s) * Dot_pixel (in m) * Step (in pixels). Then, to calculate at what speed the stimulus moves across the retina (to obtain the speed in degrees of visual angle), two factors must be taken into account: (1) the speed of the target on the screen, in meters per second, and (2) the viewing distance (usually in m): Speed on the retina = arc tan (on screen_speed / Viewing distance). For example, given a target speed on the screen of 0.420 m/s and a viewing distance of 2 m, the speed would be 11.86°/s of visual angle. The DVA is then expressed as the smallest detail that the observer is able to perceive at that speed.

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[1] * Mailing address: Universitat de Barcelona, Facultad de Psicología, Passeig de la Vall d’Hebron, 171, Barcelona, 08035, Spain. Phone: +34 933 125 145; Fax: +34 934 021 363. E-mail: jaznar2@ub.edu