Historically, stock options have been granted with an exercise price equal to the market value of the common stock on the date of grant, expire 10 years from the date they are granted, and generally vest over a one to three-year period for employees and members of the Board. Upon exercise, new shares will be issued by the Company. The restricted shares and restricted stock units (“RSUs”) granted to Board members and employees generally have a three-year graded vesting period and are subject to accelerated vesting and forfeiture under certain circumstances. In the second quarter of 2020, the Company issued performance-based options, which requires the Company to assess the likelihood of achieving certain performance milestones on a quarterly basis; approximately $0.3 million in stock compensation expense is expected to be incurred over the amortization period for these options.

The following table provides the weighted average assumptions used in determining the fair value of the stock option awards granted during the six month periods ended June 30, 2020 and 2019.

The Company recognized approximately $0.4 million and $0.5 million of stock-based compensation expense during the three-month periods ended June 30, 2020 and 2019, respectively, and approximately $0.8 million and $1.0 million for the six-month periods ended June 30, 2020 and 2019, respectively.